The correct answer is Vitamin C.

- Vitamin C, also known as ascorbic acid, plays a crucial role in the biosynthesis of collagen.
- Collagen is the primary structural protein found in connective tissues such as skin, cartilage, bones, and blood vessels.
- The formation of collagen requires the hydroxylation of proline and lysine residues, a process that depends on the presence of vitamin C as a cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase.
- This hydroxylation is essential for the stability and cross-linking of the collagen triple helix structure.

- Without adequate vitamin C, collagen synthesis is impaired, leading to symptoms such as weakened connective tissues, delayed wound healing, and in severe cases, scurvy—a disease characterized by bleeding gums, bruising, and poor wound repair.

In contrast, vitamins A, D, and K have different primary roles related to vision, calcium metabolism, and blood clotting, respectively, and do not have a direct role in collagen biosynthesis.

Key points:
- Vitamin C is essential for hydroxylation of proline and lysine in collagen.
- This process stabilizes the collagen triple helix structure.
- Deficiency of vitamin C leads to impaired collagen formation and symptoms of scurvy.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 40 / Page 1085