**Core Concept**
Vitamin K plays a crucial role in the synthesis of blood clotting factors in the liver. It is essential for the post-translational modification of these proteins, specifically the gamma-carboxylation of glutamic acid residues.

**Why the Correct Answer is Right**
Vitamin K-dependent proteins include prothrombin (factor II), factors VII, IX, and X, as well as proteins C and S. The gamma-carboxylation process is mediated by the enzyme vitamin K-dependent carboxylase, which requires vitamin K as a cofactor. This modification is necessary for the binding of calcium ions to these proteins, enabling them to interact with phospholipid surfaces during the coagulation cascade.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamin K is not involved in the synthesis of blood clotting factors. Instead, it is involved in the synthesis of clotting factors.
**Option B:** This option is incorrect because vitamin B12 is involved in DNA synthesis and the metabolism of fatty acids and amino acids, but not in the synthesis of blood clotting factors.
**Option C:** This option is incorrect because vitamin C is essential for collagen synthesis and iron absorption, but not in the synthesis of blood clotting factors.

**Clinical Pearl / High-Yield Fact**
Warfarin, a vitamin K antagonist, is commonly used as an anticoagulant to prevent thrombosis. It works by inhibiting the gamma-carboxylation process, leading to a decrease in the production of vitamin K-dependent clotting factors.

**Correct Answer: C. Vitamin K.**