Ans. is 'a' i.e.. Methylene THFFolic acido The active form of folic acid (pteroyl glutamate) is tetrahydrofolate (THF).o THF serves as a carrier of one-carbon (C1) unitQ during several biosynthetic reactions like nucleic acidQ and amino acid metabolism.o 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.o However, folic acid is more versatile than either of these two because it can transfer the C1 units in several oxidation states.o THF acts as a carrier of one carbon unitQ.o The one carbon units can be : Methyl (CH3) methylene (CH2), methenyl (CH),formyl (CHO), or formimino (CH =NH)o One carbon unit binds to THF through A5 or N10 or both A5, N10position.o For example, if formyl unit is attached to N-5, it is called N5-formyl THF; if methylene unit is attached to both N5 and Nro, it is called N5, N10 methylene THF.o Carbon units attached to N5 are formyl, formimino, or methyl; attached to N10 is formyl; and attached to both N5-N10 are methylene or methenyl.o So, various possible THF are : - N5-Formyl THF, N5-formimino THF, N5-methyl THF, Nl0-formyl THF, N5-Nl0-methylene THF and N5-N10-methenyl THF.o THF receives the C, units from various donor molecules during catabolic reactions and can transfers them tospecific acceptors for the synthesis of various compounds.o 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 important source ofsubstitutedfolates for biosynthetic reactions, and activity' of serine hydroxymethyltransferase is regulated by the state offolate substitution and availability of folate