Tsumego (Capture) #5309416 (1-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is B1. The lone white stone in the corner (A1) is already blocked at A2, so its only liberty is B1.
Playing B1 removes that last liberty and captures the stone. A stone with one liberty (in atari) is taken simply by filling that point.