xela: This is an interesting example of something than can be either a two-stage ko (after winning the first ko, black plays 'a') or an approach ko (black wins the ko then plays 'b'). Either way, black needs to win the ko twice. Note that if black 'c' then white 'b'.
I think the two-stage ko is the better way for black.
unkx80: This is a direct ko, not a two-stage ko.
xela: Ah, silly me. After black wins the ko and plays 'b', there's no need to win the ko again: the eye at 'd' is false. Thanks.