Which of the following is a type of MAO inhibitor drug?

Phenelzine is a type of monoamine oxidase inhibitor (MAOI), which is a class of drugs used primarily in the treatment of depression. MAO inhibitors function by blocking the action of monoamine oxidase, an enzyme responsible for breaking down neurotransmitters such as serotonin, norepinephrine, and dopamine in the brain. By inhibiting this enzyme, MAOIs increase the levels of these neurotransmitters, which can lead to an improvement in mood and alleviation of depressive symptoms.

The other options are different classes of antidepressants. Amitriptyline is a tricyclic antidepressant (TCA), which also affects neurotransmitter levels but through a different mechanism. Sertraline and citalopram are both selective serotonin reuptake inhibitors (SSRIs) that work by preventing the reabsorption of serotonin in the brain, thus increasing its availability. While these medications are effective for depression, they do not belong to the MAOI category, making phenelzine the only correct answer as an MAO inhibitor.