Can someone help me with cell biology?

This is easy! Ubiquitin is a highly-conserved regulatory protein that is ubiquitously expressed in eukaryotes and is responsible for the post-translational modification of a protein The most prominent function of ubiquitin is labeling proteins for proteasomal DEGRADATION. If ubiquitin cannot be attached to the proteins, then they cannot be degraded. Since you need protein levels to drop for cell division to occur, ubiquitin must be functioning properly. So, C!