Which of the following Vitamin deficiency is responsible for functional Folate deficiency ?

Vitamin B12 Deficiency Causes Functional Folate Deficiency--the "Folate Trap" When acting as a methyl donor, S-adenosyl methionine forms homocysteine, which may be remethylated by methyltetrahydrofolate catalyzed by methionine synthase, a vitamin B12 -dependent enzyme. As the reduction of methylene-tetrahydrofolate to methyl-tetrahydrofolate is irreversible and the major source of tetrahydrofolate for tissues is methyltetrahydrofolate, the role of methionine synthase is vital, and provides a link between the functions of folate and vitamin B12. Impairment of methionine synthase in vitamin B12 deficiency results in the accumulation of methyltetrahydrofolate that cannot be used -- the "folate trap." There is, therefore, a functional deficiency of folate, secondary to the deficiency of vitamin B12. Reference: Harper; 30th edition; Page no: 560