Kyu Exercise 137 / Attempts

It's the only move  

If B doesn't play at a, W taking it will have an eye in the corner and since the two black stones on the edge can't escape, the second eye under them.

W can't ignore  

W can't ignore else he doesn't have enough space for two eyes.

W can't ignore  

But preventing Black from taking the 2 stones in the corner also leaves him with dead shape.

Dead  

unkx80: Correct!