**Core Concept**
Vitamin K plays a crucial role as a cofactor in the post-translational modification of proteins, particularly in the gamma-carboxylation of glutamate residues. This process is essential for the activation of various proteins involved in blood clotting, bone metabolism, and other physiological functions.

**Why the Correct Answer is Right**
Vitamin K-dependent carboxylases, specifically gamma-glutamyl carboxylase, utilize Vitamin K as a cofactor to facilitate the conversion of glutamate residues to gamma-carboxyglutamate (Gla) residues in target proteins. This modification is vital for the biological activity of these proteins, including prothrombin and other clotting factors. The Vitamin K-dependent carboxylation process involves the reduction of Vitamin K to its hydroquinone form, which then donates a carboxyl group to the glutamate residue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Vitamin K is not a cofactor in the synthesis of cholesterol.
**Option B:** This option is incorrect as Vitamin K is not directly involved in the regulation of gene expression through histone modification.
**Option C:** This option is incorrect as Vitamin K is not a cofactor in the synthesis of DNA or RNA.

**Clinical Pearl / High-Yield Fact**
Vitamin K deficiency or warfarin therapy can lead to impaired clotting factor activity, resulting in an increased risk of bleeding. Patients on warfarin require regular monitoring of international normalized ratio (INR) to ensure therapeutic anticoagulation.

**Correct Answer: D. Vitamin K-dependent carboxylation of glutamate residues**