Which of the following compounds is not an antacid?

We begin by recalling the definition of an antacid. An antacid is a medicinal substance that neutralises excess gastric $$\mathrm{HCl}$$ in the stomach, thereby relieving acidity and heart-burn. Substances used for this purpose are usually basic hydroxides, carbonates, or specifically designed histamine-H2 receptor blockers that reduce acid secretion.

Now we examine each option one by one.

First we look at ranitidine. Ranitidine belongs to the class of H2 receptor antagonists. These drugs block the action of histamine on the gastric parietal cells and thus reduce the volume and concentration of gastric acid. Because lowering gastric acidity directly combats hyperacidity, ranitidine is indeed used as an antacid. So, option A is an antacid.

Next, aluminium hydroxide. Aluminium hydroxide is a basic hydroxide, typically written as $$\mathrm{Al(OH)_3}$$. The neutralisation reaction with stomach acid is

$$\mathrm{Al(OH)_3 + 3\,HCl \rightarrow AlCl_3 + 3\,H_2O}$$

Because it reacts with and neutralises $$\mathrm{HCl}$$, aluminium hydroxide is employed directly as an antacid. Hence, option B is also an antacid.

Now consider cimetidine. Cimetidine, like ranitidine, is an H2 receptor blocker. By reducing the secretion of gastric acid, it serves the same therapeutic purpose as an antacid even though its mechanism differs from simple neutralisation. Therefore, option C is classified as an antacid.

Finally we look at phenelzine. Phenelzine is a monoamine-oxidase inhibitor (MAOI) used in the treatment of depression. Its pharmacological action is unrelated to gastric acid neutralisation or suppression. Because it neither neutralises stomach acid nor blocks its secretion, it is not an antacid.

Comparing all four options, only phenelzine fails to meet the functional definition of an antacid.

Hence, the correct answer is Option D.