**Core Concept**
Vitamin K plays a crucial role in the post-translational modification of proteins, specifically in the gamma-carboxylation of glutamic acid residues to form gamma-carboxyglutamic acid (Gla). This process is essential for the activation of various proteins involved in blood coagulation, such as prothrombin and clotting factors II, VII, IX, and X.

**Why the Correct Answer is Right**
The gamma-carboxylation reaction involves the enzyme vitamin K-dependent carboxylase, which requires vitamin K as a cofactor. Vitamin K donates a carboxyl group (-COOH) to the glutamic acid residue, resulting in the formation of Gla. This modification is necessary for the binding of calcium ions, which is essential for the proper functioning of these proteins. The vitamin K-dependent carboxylation reaction is a critical step in the activation of these proteins, allowing them to bind to phospholipid surfaces and facilitate blood coagulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydroxylation is a different post-translational modification that involves the addition of a hydroxyl group (-OH) to an amino acid residue. This process is not dependent on vitamin K.
**Option B:** Chelation is a process that involves the binding of metal ions to specific molecules, and it is not directly related to vitamin K or its role in protein modification.
**Option C:** Transamination is a process that involves the transfer of an amino group (-NH2) from an amino acid to a keto acid, resulting in the formation of a new amino acid. This process is not dependent on vitamin K.

**Clinical Pearl / High-Yield Fact**
The vitamin K-dependent carboxylation reaction is a key step in the activation of blood clotting factors, and deficiencies in vitamin K can lead to bleeding disorders such as hemophilia.

**✓ Correct Answer: D. Carboxylation. Vitamin K is required for the gamma-carboxylation of glutamic acid residues to form gamma-carboxyglutamic acid (Gla).**