Beginner Exercise 94 / Solution

Ko  

White's only chance of survival is starting ko by . It is atari to the Black string and force him to fight the ko.

(White can't wait for Black's approach, say a, to start the ko - Black will play at instead)

good when outside is alive  

would be the best move to kill Black if White's outside string were alive. Here it's a capturing race and White is far behind - Black has 8 liberties while White has only 3. (See Big eye liberties for reference).