Which of the following compounds is not an antacid?

Antacid: the medicine used for acidity is antacids. Aluminium hydroxide, Cimetidine, and ranitidine are used for acidity and so are antacids.

Whereas Phenelzine is not antacid, it is an antidepressant drug.

Hence the correct answer is C i.e. Phenelzine.