Final answer:
Cdk inhibitor proteins block the assembly/activation of Cdk complexes, with these inhibitors ensuring that conditions are right for the cell cycle to proceed. Cyclins regulate the cycle by binding and activating Cdks, but inhibitor proteins can override this if necessary.
Step-by-step explanation:
The molecule that blocks the assembly/activation of Cdk complexes is C. Cdk inhibitor proteins. Cyclins are regulatory molecules that determine the forward momentum of the cell cycle, while Cdk inhibitors are molecules that prevent the full activation of Cdks.
These inhibitor molecules oversee and monitor specific cell cycle events, and the block on Cdks will only be lifted once the monitored event is completed. The p53 protein can also control the activation of Cdk inhibitor proteins, functioning as a tumor suppressor to prevent cell division when there is DNA damage.