is the only correct move. If tries to destroy the eye at the top, White can block at . With the marked stone, Black cannot play at a to connect . So is effectively captured, and White lives.
Moosky: If B plays at , W will answer at , effectively making 2 eyes. This will be the better move for Black, as it takes 1-2 points away from White (if B captures the marked stone).
Jacob4Jesus: Doesn't at kill? Is there a better play than ?
JoelR: No, If at , then at . Please read these things out, making note of which stones are still on the board and which are not.