**Core Concept**
Folic acid plays a crucial role in one-carbon metabolism, which involves the transfer of one-carbon units to various substrates. This process is essential for the synthesis of nucleic acids, amino acids, and other biomolecules.

**Why the Correct Answer is Right**
Folic acid acts as a coenzyme for several enzymes involved in one-carbon metabolism, including dihydrofolate reductase (DHFR) and thymidylate synthase. In these reactions, folic acid donates one-carbon units, which are then used to synthesize thymidine (a nucleotide) and other biomolecules. The tetrahydrofolate (THF) form of folic acid is particularly important, as it can accept and donate one-carbon units via various reactions. The deficiency of folic acid can lead to impaired one-carbon metabolism, resulting in megaloblastic anemia and other disorders.

**Why Each Wrong Option is Incorrect**
**Option B:** Thiamine is a vitamin involved in carbohydrate metabolism, particularly in the decarboxylation of alpha-keto acids in the Krebs cycle. It is not directly involved in one-carbon metabolism.
**Option C:** Biotin is a coenzyme for carboxylase enzymes, which participate in the synthesis of fatty acids and gluconeogenesis. While biotin is essential for various metabolic processes, it is not directly involved in one-carbon metabolism.
**Option D:** Niacin is a component of NAD+ and NADP+, which are involved in redox reactions and energy metabolism. Niacin is not directly involved in one-carbon metabolism.

**Clinical Pearl / High-Yield Fact**
Folic acid deficiency can lead to megaloblastic anemia, which is characterized by large, immature red blood cells. This condition is often seen in individuals with malabsorption syndromes, such as celiac disease, or those taking certain medications, like methotrexate.

**✓ Correct Answer: A. Folic acid**