Final answer:
Protein phosphorylation is commonly involved in signal transduction, a process where kinases add phosphate groups to proteins, changing their function and leading to various cellular responses.
Step-by-step explanation:
The cellular process that commonly involves protein phosphorylation is C) Signal transduction. During signal transduction, kinase enzymes phosphorylate proteins, altering their structure and function, thereby activating or inactivating them. This process is part of a complex series of events within a cell that leads to a specific cellular response to an external signal. For example, signal transduction can lead to changes in gene expression, cell metabolism, growth, or even cell death. The termination of such signaling pathways is achieved by the action of phosphatases, which remove the phosphate groups from proteins, a process known as dephosphorylation.