Final answer:
The proteolysis of M and S cyclins during mitosis is carried out by B) proteasomes, which are responsible for degrading proteins tagged with ubiquitin.
Step-by-step explanation:
The proteolysis of M and S cyclins midway through mitosis is accomplished by proteasomes.
These large protein complexes degrade proteins that have been tagged with ubiquitin, a small protein that serves as a signal for protein degradation. Ubiquitin attaches to proteins and marks them for destruction by the proteasome, which then breaks down the protein into small peptides.