Before presenting the solution proper, it is worth nothing that aims at in this diagram. This is going to spell trouble for Black, the result is at least a favourable ko for White. So Black at a definitely is not going to be a solution.
Bill: When White can win the ko, Black will avoid it with .
Bill: Late in the game (where this position is likely to arise), this is the way for White to play if Black can win the ko.
By now, it should be clear that is the only solution. See that White a means self-atari.
If , then can play elsewhere.
unkx80: Not necessary a good idea. White can just play elsewhere, and then locally White has one more ko threat than before.
Eaglehelm: Not even a ko. The move captures two stones.