Shortage of liberties prevents White from playing at
. Thus White is left with a false eye.
Black does not play at
for a peep because White would play at
and live. (
and
are miai for the second eye.)
captures one stone, but
leaves White dead. The points a and b are miai, meaning that the eye is false.
fringd: simply taking point b in the above diagram was my instinct. This move seems good, but a ko is failure in this situation.
Black must play and
in the order described above.
Black is a thank you move that allows White to connect at
and live.