a) White to mate in one.

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b) White to mate in one.

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c) White to mate in two.

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d) White to mate in four.

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Solutions:  a) N-g7 mate            b) N-f7 mate              c) Q-g8ch, RxQ    followed by  N-f7 mate

                      d) The immediate Q-g8ch will not work in this case, since black can take the queen with the                                    king instead of the rook. Instead, white must first play N-f7ch, K-g8, then  N-h6ch, K-h8.                                    Now white can play Q-g8ch.  The rook has to take the queen as the king cannot put itself                                    into check, and now, finally, N-f7 mates!