This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
B12 is needed in which step of folate synthesis 
Select and explain. 
A. Tetrahydrofolate to folinic acid, B. Methyl tetrahydrofolate to tetrahydrofolate, C. Formyl tetrahydrofolate to tetrahydrofolate, D. Methylene tetrahydrofolate to methyltetrahydrofolate
B: Methyl tetrahydrofolate to tetrahydrofolate
Answer: B: Methyl tetrahydrofolate to tetrahydrofolate 
Explanation: Vit B12 act as a coenzyme to enzyme homocysteine methyltransferase in the reaction THFA is the carrier of One-Carbon groups The folic acid is first reduced to 7,8-dihydrofolic acid and fuher reduced to 5,6,7,8-tetrahydrofolic acid (THFA) . Both reactions are catalyzed by NADPH dependent folate reductase. The THFA is the carrier of one-carbon groups. One carbon compound is an organic molecule that contains only a single carbon atom. The following groups are one carbon compounds: Formyl (-CHO) Formimino (-CH=NH) Methenyl (-CH=) Methylene (-CH2-) Hydroxymethyl (-CH2OH) Methyl (-CH3). One carbon groups bind to THF through N5 are Formyl, Formimino or methyl N10 are Formyl Both N5 and N10 are Methylene and Metheny Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 475