Final answer:
APC/C is the protein that destroys M-cyclin at the end of M phase.
Step-by-step explanation:
The protein that destroys M-cyclin at the end of M phase is APC/C (anaphase promoting complex/cyclosome).
APC/C is a large protein complex that controls the progression of the cell cycle by marking specific proteins, including M-cyclin, for degradation by the proteasome. During the transition from metaphase to anaphase, APC/C becomes activated and targets M-cyclin for destruction, leading to the exit of the cell from M phase.
Therefore, the correct answer is a) APC/C.