CDKs are important in the regulation of the cell cycle. They carry out their function by:

a. adding phosphate groups to target proteins.

b. removing phosphate groups from target proteins.

c. preventing the progression of a cell from one stage of the cell cycle to the next.

d. degrading cyclin proteins.

A. Adding phosphate groups to target proteins

Explanation:

CDKs (cyclin-dependent kinases) are the protein kinases involved in regulation of the cell cycle. Cyclins are the regulatory proteins that serve to activate the cyclin-dependent kinases. The activity of various Cdks increases and then decreases as the cell proceeds through the cell cycle.

Binding of a specific CDK with its corresponding cyclin results in formation of a cyclin-Cdk complex. Cyclin-Cdk complexes serve to phosphorylate the target enzymes and other proteins. Phosphorylation activates some of these proteins while deactivating the others. For example, the protein p27 is degraded upon phosphorylation. Since p27 is one of the major inhibitor of cell division, the reduced levels of p27 makes the non-dividing cell s to enter the cell division.