For White, is best. forms a bent four and lives, and White can play at a to capture the stones at the corner.
and is probably the strongest attempt to kill. The main thing to see in this problem is the tesuji at , which makes and miai for life.
is wrong. White makes a placement at , followed by an atari at on the entire Black group which forces to capture, finally a throw-in at which makes the eye at the corner a false eye.