Ans. is 'a' i.e., Methylene THF Folic acid The active form of folic acid (pteroyl glutamate) is tetrahydrofolate (THF). THF serves as a carrier of one-carbon (C1) unit0 during several biosynthetic reactions like nucleic acidQ and amino acid metabolism. Two other cofactors are also known to be involved in the addition of one carbon (C1) unit to a metabolic precursor, biotin in carboxylation reaction and S-adenosylmethionine (SAM) as methylating agent. However, folic acid is more versatile than either of these two because it can transfer the C, units in several oxidation states. THF acts as a carrier of one carbon unit0. The one carbon units can be : Methyl (C.113), methylene (CH2), methenyl (CH), formyl (CHO), orformimino (CH =NH). One carbon unit binds to THF through N5 or N1deg or both N5, N'deg position. For example, if formyl unit is attached to N-5, it is cal led N5-formyl THF; if methylene unit is attached to both N5 and 1\l'deg, it is called N5, N'deg methylene THF. Carbon units attached to N5 are formyl, formimino, or methyl; attached to N'deg is formyl; and attached to both N5-N10 are methylene or methenyl. So, various possible THF are : - N5-Formyl THF, N5-formimino THF, N5-methyl THF, N1deg-formyl THF, N5-N10methylene THF and N5-N'deg-methenyl THF. THF receives the C, units from various donor molecules during catabolic reactions and can transfers them to specific acceptors for the synthesis of various compounds. The major point of entry for one-carbon unit into substituted folates is methylene THF, which is formed by the reaction of glycine, serine and choline. Serine is the most impoant source of substituted folates for biosynthetic reactions, and activity of serine hydroxymethyltransferase is regulated by the state of folate substitution and availability of folate.