**Core Concept**
Serine is an amino acid that can undergo phosphorylation, a post-translational modification where a phosphate group is added to the amino acid. This process is crucial in various cellular signaling pathways, including protein kinase cascades and glycogen metabolism.

**Why the Correct Answer is Right**
Serine contains a hydroxyl group (-OH) in its side chain, which makes it susceptible to phosphorylation. The enzyme responsible for adding a phosphate group to serine is a kinase, specifically a protein kinase. This phosphorylation event can alter the activity, location, or binding affinity of the target protein. For instance, the phosphorylation of serine residues in the glycogen synthase enzyme activates it, promoting glycogen synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Cysteine is an amino acid that can undergo other post-translational modifications, such as disulfide bond formation, but it is not typically phosphorylated.
**Option B:** Leucine is a non-polar, hydrophobic amino acid that does not possess a hydroxyl group, making it unsuitable for phosphorylation.
**Option C:** Methionine is an essential amino acid that can undergo other modifications, like methylation, but it is not commonly phosphorylated.

**Clinical Pearl / High-Yield Fact**
Phosphorylation of serine residues is a key regulatory mechanism in many cellular processes, including protein-protein interactions, enzyme activation, and signal transduction pathways. This modification is reversible, allowing for dynamic control of protein function.

✓ Correct Answer: D. Serine