No suicide & ko
Two rules hold the whole game together. First, no suicide: a stone may not be played where it would have no liberties — unless the same move captures enemy stones and frees the point, in which case the capture happens first and the play is legal. Second, no immediate repetition: after a ko capture, the opponent may not recapture on the very next move, but must first play elsewhere as a ko threat; only after you answer may the recapture become legal (the full story returns in 6.10 and 8.8). On this board both rules are live at once: the lone white stone has one liberty left, while the empty point inside the white ring at the top is suicide for Black — the rules simply refuse it. Black to play: take the capture that works.