**Core Concept**
The transfer of 1-carbon units is a crucial process in the synthesis of purines, pyrimidines, and amino acids. This process involves the donation of a 1-carbon unit to a recipient molecule, which is essential for various cellular functions, including DNA synthesis and repair.

**Why the Correct Answer is Right**
The vitamin required for the transfer of 1-carbon units is **Folic Acid (Vitamin B9)**. Folic acid is a coenzyme for the enzyme dihydrofolate reductase (DHFR) and is necessary for the conversion of dihydrofolate to tetrahydrofolate, which serves as a carrier of 1-carbon units. Tetrahydrofolate donates 1-carbon units to various molecules, such as thymidylate synthase, which is essential for DNA synthesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Riboflavin (Vitamin B2) is a coenzyme for flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), but it is not involved in the transfer of 1-carbon units.
* **Option B:** Thiamine (Vitamin B1) is a coenzyme for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, but it is not involved in the transfer of 1-carbon units.
* **Option D:** Pyridoxine (Vitamin B6) is a coenzyme for various enzymatic reactions, including amino acid metabolism, but it is not involved in the transfer of 1-carbon units.

**Clinical Pearl / High-Yield Fact**
Folic acid deficiency can lead to megaloblastic anemia, which is characterized by the production of large, immature red blood cells. This condition can be treated with folic acid supplementation, highlighting the importance of this vitamin in maintaining normal hematopoiesis.

**Correct Answer:** C. Folic Acid