White to move. Find the one move that wins. About 1000 ELO.
Black's own pieces surround the king. A knight finishes it.
Which piece can check a king that's completely surrounded?
A move that attacks the king with no way out: the game ends on the spot.
Look at every check first. Count the king's escape squares and ask which check covers all of them.
Knights jump over pieces, so a king hemmed in by its own army is vulnerable to a knight check.
Solve it on the board above, or take a hint. Every ChessLvl puzzle is checked by a chess engine to have exactly one best move.