its a tossup between 'a' and 'b' for me. i would probably play 'a' but i tend to be too aggressive. 'c' doesn't look promising.

If 'a' works, then I think it should be played to keep up the pressure (ha!). Even if it doesn't completely »work«, it might be worthwhile to play it as a probe.

It is unclear if defending at 'b' is absolutely urgent, but Black certainly has various options to exploit the thinness here.

Black has no sente plays around 'c'. White might want to play at 'c' before defending at 'b' though, but one would have to look deeply what options Black really has to answer there. Leaving 'a' aside, 'c' would again stress the opponent.

So, 'a' for me right now.

Let's figure out the continuation of the cut.

First instinct: cut and extend. This doesn't work.

Click Here To Show Diagram Code

[go]$$Bc Oops
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . O . . . . . . . . . . . O . . |
$$ | . . O , . . . . . , . . O . b O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X c . |
$$ | . . . O . . . . . X . . . X . X O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . 4 . . . . . . . . . . . |
$$ | . . O O X . 1 2 . . . . . . . . . . . |
$$ | . . X O X . O 3 X . . . . . . . . . . |
$$ | . . O X X O 5 . . . X . . . . . . . . |
$$ | . . O X . . . O . O . . X X X X X X . |
$$ | . . O X . . X O . . . O O X O O O O . |
$$ | . . . . . . . . . . . . . O . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Second attempt: cut and atari. Black can still connect by ko. Black takes first.

Click Here To Show Diagram Code

[go]$$Bc ko
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . O . . . . . . . . . . . O . . |
$$ | . . O , . . . . . , . . O . b O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X c . |
$$ | . . . O . . . . . X . . . X . X O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . 4 . . . . . . . . . . . . |
$$ | . . O O X 6 1 2 . . . . . . . . . . . |
$$ | . . X O X 7 O 3 X . . . . . . . . . . |
$$ | . . O X X O 5 8 . . X . . . . . . . . |
$$ | . . O X . . . O . O . . X X X X X X . |
$$ | . . O X . . X O . . . O O X O O O O . |
$$ | . . . . . . . . . . . . . O . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Third attempt: the other atari.

Click Here To Show Diagram Code

[go]$$Bc Oops
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . O . . . . . . . . . . . O . . |
$$ | . . O , . . . . . , . . O . b O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X c . |
$$ | . . . O . . . . . X . . . X . X O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . 6 5 . . . . . . . . . . . . |
$$ | . . O O X 4 1 2 . . . . . . . . . . . |
$$ | . . X O X . O 3 X . . . . . . . . . . |
$$ | . . O X X O . . . . X . . . . . . . . |
$$ | . . O X . . . O . O . . X X X X X X . |
$$ | . . O X . . X O . . . O O X O O O O . |
$$ | . . . . . . . . . . . . . O . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

I can't read the continuation. Black's corner stones seem in difficulty, however White has also helped Black to convert his moyo into territory.

On the other hand, if White doesn't play in the upper right corner, what can Black do? The following variation reduces White to a dead L-group, so White can't play like this:

Click Here To Show Diagram Code

[go]$$Bc L group
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . 7 . . . . |
$$ | . . . . O . . . . . . . 9 5 4 8 O . . |
$$ | . . O , . . . . . , . 0 O 6 3 O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

So probably White must let Black cut his group; White won't have comfortable life in the corner.

Click Here To Show Diagram Code

[go]$$Bc cut
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . 7 6 . . . . |
$$ | . . . . O . . . . . . . . 5 4 . O 8 . |
$$ | . . O , . . . . . , . . O . 3 O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

Let's try "c". No idea what is the best sequence so I'll try something random.

Click Here To Show Diagram Code

[go]$$Bc Oops
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . O . . . . . . . . . . . O . . |
$$ | . . O , . . . . . , . . O . b O . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ | . . . . . . . . . . . . X . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . 7 5 . |
$$ | . . . . . . . . . . . . . . . 6 X 2 . |
$$ | . . . O . . . . . X . . . X . X O 3 . |
$$ | . . . . . . . . . . . . . . . . 4 . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O X . 1 . . . . . . . . . . . . |
$$ | . . X O X . O . X . . . . . . . . . . |
$$ | . . O X X O . . . . X . . . . . . . . |
$$ | . . O X . . . O . O . . X X X X X X . |
$$ | . . O X . . X O . . . O O X O O O O . |
$$ | . . . . . . . . . . . . . O . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Hmm... Can that live? The wall at R4 is pretty scary.

So I'd choose between "a" and "b". Hard to decide. Maybe "b" first. If Black doesn't protect then White can play "a" later, otherwise White can reduce Black's moyo instead.