In the context of phosphorylation, what is the primary function of a protein kinase?

• A. It hydrolyzes $ATP$ to generate $cAMP$ as a second messenger.
• B. It removes phosphate groups from phosphorylated proteins, reversing phosphorylation.
• C. It catalyzes the transfer of a phosphate group from $ATP$ to a specific amino acid residue on a target protein.
• D. It catalyzes the formation of peptide bonds during protein synthesis on the ribosome.

Answer: (C)