**Core Concept**
Vitamin C, also known as ascorbic acid, plays a crucial role in the hydroxylation of proline and lysine residues in collagen. This process is essential for the stabilization and cross-linking of collagen fibers, which provide strength and elasticity to connective tissue.

**Why the Correct Answer is Right**
Vitamin C acts as a cofactor for the enzyme prolyl hydroxylase, which catalyzes the hydroxylation of proline residues in collagen. This reaction is critical for the proper folding and assembly of collagen triple helices. Without sufficient vitamin C, prolyl hydroxylase activity is impaired, leading to the accumulation of unhydroxylated collagen and its associated disorders, such as scurvy.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin B6 is involved in various enzymatic reactions, including the decarboxylation of amino acids, but it is not directly involved in the hydroxylation of proline.
**Option B:** Vitamin E is an antioxidant that protects cells from oxidative damage, but it is not a cofactor for prolyl hydroxylase.
**Option C:** Vitamin K is required for the gamma-carboxylation of glutamic acid residues in clotting factors, but it is not involved in the hydroxylation of proline.

**Clinical Pearl / High-Yield Fact**
Remember that vitamin C deficiency can lead to scurvy, a condition characterized by weakened connective tissue, bleeding gums, and poor wound healing.

**Correct Answer: A. Vitamin C**