**Core Concept**
Collagen formation is a complex process involving multiple enzymes and proteins. It is essential for the integrity and strength of connective tissue in various parts of the body, including skin, bones, tendons, and ligaments.

**Why the Correct Answer is Right**
Collagen formation is initiated by the enzyme prolyl hydroxylase, which catalyzes the hydroxylation of proline residues in the procollagen molecule. This process is crucial for the stabilization of the triple-helix structure of collagen. A deficiency in vitamin C (ascorbic acid) impairs the activity of prolyl hydroxylase, leading to the production of abnormal collagen fibers that are prone to degradation. This results in a range of clinical manifestations, including scurvy, a disease characterized by weakened connective tissue, poor wound healing, and bleeding gums.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a deficiency in vitamin B6 (pyridoxine) primarily affects the synthesis of neurotransmitters and the regulation of homocysteine levels in the blood. It is not directly involved in collagen formation.

**Option B:** This option is incorrect because a deficiency in vitamin D affects bone mineralization and calcium homeostasis, leading to conditions such as rickets and osteomalacia. While vitamin D is essential for bone health, it is not directly involved in collagen formation.

**Option C:** This option is incorrect because a deficiency in vitamin K affects blood clotting and bone health, leading to conditions such as easy bruising and bleeding. Vitamin K is necessary for the activation of clotting factors and the production of osteocalcin, but it is not directly involved in collagen formation.

**Clinical Pearl / High-Yield Fact**
It is essential to note that vitamin C deficiency can lead to a range of clinical manifestations, including impaired wound healing, bleeding gums, and joint pain. This is because collagen is a critical component of the extracellular matrix, and its proper formation is essential for tissue integrity and function.

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