Tsumego (Capture) #5309418 (1-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is C1. The three white stones (A1, B1, B2) are surrounded, with C1 as their only liberty.
Even a three-stone group with a single liberty is captured by filling that one point (C1). What decides capture is the number of remaining liberties, not the number of stones.