B12 is needed in which step of folate synthesis

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