**Core Concept**
The hydroxylation of proline is a crucial step in collagen synthesis, allowing for the formation of a stable triple-helix structure. This process involves the post-translational modification of proline and lysine residues in the procollagen molecule.

**Why the Correct Answer is Right**
The correct answer is **D**. The hydroxylation of proline in collagen synthesis requires three key components:
* **Prolyl hydroxylase (P4H)**: an enzyme that specifically recognizes and hydroxylates proline residues.
* **Ascorbic acid (Vitamin C)**: a cofactor necessary for the activity of prolyl hydroxylase.
* **Fe2+ (Iron)**: a metal ion required for the enzymatic activity of prolyl hydroxylase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide any relevant information about the hydroxylation of proline.
* **Option B:** This option is incorrect because it may refer to other enzymes involved in collagen synthesis, such as lysyl hydroxylase, which hydroxylates lysine residues.
* **Option C:** This option is incorrect because it may refer to other cofactors or metal ions involved in collagen synthesis, but it is not directly related to the hydroxylation of proline.

**Clinical Pearl / High-Yield Fact**
Vitamin C deficiency can lead to impaired collagen synthesis, resulting in scurvy, a disease characterized by weakened connective tissue, bleeding gums, and poor wound healing.

**Correct Answer: D.**