This is problem 18 from goproblems.com (25k-15k level)
The correct answer according to the site is a. However I fail to see why b isn't also correct. If black b and white a, black can then continue by capturing the stone which is in atari. Why is it not a solution?
agro1986 (19k KGS)
Bill: The somewhat longer answer is yes, but.
Thanks to White gets a huge ko threat to kill Black's group.
ilan: You are all wrong! The right move is c.
Xanthus: Why would this be better than a? W still has the ko-threat (or even 2 threats if B responds to taking of c with b). I'm not even sure if it is better than b, because it is a point worse if white defends, even though it might generate a (small) ko-threat for B.
Bill: I think this is an example of Ilan's wit. Good one, Ilan! ;-)