Which of the following can occur many times during a protein's life span?

A. Disulfide bond formation
B. Gamma-carboxylation
C. Glycosylation
D. Phosphorylation
E. Proteolys

Is it D. Phosphorylation ?

Good!

The correct answer is D. A variety of co- and post-translational modifications can occur with proteins, including all those listed. Most of these occur only at one point during the formation of a functional protein, but protein phosphorylation (by addition of phosphate by protein kinases), and then later dephosphorylation, can occur many times during an individual protein molecule's life span. Protein phosphorylation is commonly used by the cell to either upregulate or downregulate a protein's activity.
Disulfide bond formation (choice A) occurs during protein formation to stabilize connections between different parts of a protein, so that the protein will not destabilize after cleavage of peptide bonds.

Gamma-carboxylation (choice B) produces sites that bind calcium ions, and is particularly common among blood clotting factors.

Glycosylation (choice C) is the addition of oligosaccharide as proteins pass through the ER and Golgi apparatus, and is often used by the cell to help direct proteins to particular destinations.

Proteolysis (choice E) is the cleavage of peptide bonds to remodel proteins and activate them. Examples of proteins that undergo proteolysis include proinsulin, trypsinogen, and prothrombin.