Which of the following compounds is not an antacid?
(a)- Aluminium hydroxide
(b)- Cimetidine
(c)- Phenelzine
(d)- Ranitidine

Hint:Our stomach produces hydrochloric acid which helps in digestion of food, without harming the stomach. During indigestion, our stomach produces excess hydrochloric acid which causes pain and irritation. To get rid of this pain, an individual often takes an antacid.

Complete step by step answer:
Antacid is commonly known as antacid. It neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach. Antacids work on the principle of neutralization reaction. They contain alkalis such as carbonates, aluminium hydroxide, magnesium hydroxide, sodium, etc. When an individual intake an antacid, he/she intakes alkali, which reacts with excess acid present in our stomach, producing salt and water. Because of this neutralization reaction, the person gets relief. Magnesium hydroxide (milk of magnesia) is a mild base, which is often used for this purpose.

(a)- Aluminum hydroxide is a basic inorganic salt. It is slowly solubilized in the stomach and reacts with hydrochloric acid to form aluminum chloride and water, as a result neutralizing the hydrochloric acid.

(b)- Cimetidine is a drug, used to reduce the excess acid content in our stomach. It is often used to treat and prevent certain types of stomach ulcer, gastroesophageal reflux disease (GERD). These conditions arise when stomach acid backs up into the esophagus and causes heartburn. It is available in the market under the brand name Tagamet.

(c)- Phenelzine is an antidepressant. This medication treats depression by restoring the balance of certain natural substances (neurotransmitters) in the brain. It can also improve our mood and feelings. It is commonly available in the market under the brand name Nardil.

(d)- Ranitidine is available in the market under the brand name Zantac. It is a medication that reduces stomach acid production. Its most common use involves treatment of peptic ulcers, gastroesophageal reflux disease.

So, the correct option is (c).

Note: Antacid was first created in 1928 by pharmacist James Howe to treat his wife's indigestion. Later in the year 1930, it became popular in American market. Aluminium hydroxide is sometimes also prescribed to people with certain kidney diseases.