**Core Concept**
Collagen synthesis is a complex process involving the action of various enzymes and proteins. It requires the presence of specific nutrients and vitamins to facilitate the hydroxylation of proline and lysine residues, which are crucial for the formation of stable hydrogen bonds between collagen molecules.

**Why the Correct Answer is Right**
Vitamin C plays a critical role in collagen synthesis by serving as a co-factor for prolyl hydroxylase and lysyl hydroxylase, enzymes responsible for the hydroxylation of proline and lysine residues. This process is essential for the stabilization of collagen molecules and the formation of a strong, stable collagen triple helix. Without vitamin C, collagen molecules are prone to degradation, leading to conditions such as scurvy.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin A is not directly involved in collagen synthesis, although it does play a role in maintaining healthy skin and mucous membranes.
**Option B:** Vitamin E is an antioxidant that helps protect cells from damage, but it is not essential for collagen synthesis.
**Option C:** Vitamin K is necessary for blood clotting and bone health, but it is not directly involved in collagen synthesis.

**Clinical Pearl / High-Yield Fact**
Vitamin C deficiency can lead to scurvy, a condition characterized by fatigue, malaise, and the development of collagen-deficient connective tissue.

**Correct Answer: D. Vitamin C.**