There's one eye in the corner already, so the problem is basically how to destroy the potential eye in the middle.
is the key point here.
One defense would be 'a' and 'b', as Black can play 'a' next to capture the three stones.
After , there are two potential eyes at around 'a' and 'b'.
Since 'c' and 'd' are miai to destroy the eye at 'a', we start with the one on the right.