<prompt> Which vitamin is required for transfer of 1-carbon unit?Vitamin AFolic acidVitamin B12Niacin  <response>: Tetrahydrofolate can carry one-carbon fragments attached to N-5 formyl, formimino, or methyl groups, N-10 formyl or bridging N-5-N-10 methylene or methenyl groups. 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharmaceutically known as folinic acid, and the synthetic racemic compound leucovorin.The major point of entry for one-carbon fragments into substituted folates is methylene-tetrahydrofolate. 