Final answer:
A) Phosphorylation is the most common method for activating/inactivating proteins involved in cell cycle activities, typically carried out by protein kinases like Cdks that modify target proteins by adding phosphate groups.
Step-by-step explanation:
The most common method for activating/inactivating proteins required for cell cycle activities is Phosphorylation. Protein kinases, such as Cyclin-dependent kinases (Cdks), play a key role in this process by transferring a phosphate group from ATP to specific amino acids on the target protein. This modification can either activate or inactivate the enzyme.
For instance, the phosphorylation of serine, threonine, and tyrosine residues can change the activity of an enzyme or create a binding site for further interactions in a signaling pathway. The activated Cdk/cyclin complex is a critical regulator that can phosphorylate other proteins to advance the cell to the next phase of the cycle.