**Core Concept**
Decarboxylation reactions involve the removal of a carboxyl group (-COOH) from a molecule, often resulting in the release of carbon dioxide. These reactions are crucial in various metabolic pathways, including amino acid metabolism. Vitamins can act as coenzymes, facilitating these reactions by donating or accepting electrons.

**Why the Correct Answer is Right**
Vitamin B6 (pyridoxine) plays a significant role in decarboxylation reactions as a coenzyme. It is involved in the decarboxylation of amino acids, such as histidine to histamine, and glutamate to gamma-aminobutyric acid (GABA). Vitamin B6 acts by forming a covalent bond with the substrate, facilitating the removal of the carboxyl group and the subsequent reaction. The pyridoxal phosphate (PLP) form of vitamin B6 is the active coenzyme in these reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) is primarily involved in the decarboxylation of alpha-keto acids in the Krebs cycle, not as a coenzyme in decarboxylation reactions.
**Option B:** Riboflavin (Vitamin B2) acts as a coenzyme for flavin-containing enzymes, primarily involved in redox reactions, not decarboxylation.
**Option C:** Niacin (Vitamin B3) is involved in redox reactions as a component of NAD and NADP, not decarboxylation reactions.

**Clinical Pearl / High-Yield Fact**
Vitamin B6 deficiency can lead to impaired amino acid metabolism, resulting in increased levels of homocysteine and methylmalonic acid. This can have serious consequences, including neurological symptoms and increased risk of cardiovascular disease.

**Correct Answer: C. Vitamin B6**