Block the attack
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In this level, your opponent is attacking one of your pieces. You must block the attack.
Blocking the attack means that you put one of your pieces between the attacking piece, and the piece being under attack.
The white rook on b8 is attacking black’s queen on d8.
Black's queen cannot run away, as that would leave the black king in check (the queen is "pinned").
Capturing the rook with Qxb8 would lose the queen to Bxb8.
However, black can block the attack with the correct move 1...Rc8!
This is possible only if the attacking piece has a long-range attack. This means the attacking piece must be a queen, rook, or bishop. It cannot be a pawn, king, or knight.
The blocking piece must be less valuable than the piece being attacked, otherwise the opponent could just capture the blocking piece.
The blocking piece should also not be more valuable than the attacking piece, otherwise the opponent would still make a material gain by capturing the blocking piece - although this option might still be preferable to minimize losses, if the blocking piece is of less value than the attacked piece.