The deficiency of which of the following vitamins impairs the metabolism of neurotransmitters in the body leading to serious disorders like depression?

Low levels of  vitamin B9 can disrupt transmethylation reaction, leading to an accumulation of homocysteine (hyperhomocisteinemia) and to impaired metabolism of neurotransmitters (especially the hydroxylation of dopamine and serotonin from tyrosine and tryptophan), phospholipids, myelin and receptors. All of these can lead to the development of various disorders including depression.