'X' is a chemical present in our stomach and is necessary for digestion of…

Concept: The gastric glands in the stomach lining secrete hydrochloric acid to activate the digestive enzyme pepsin and to kill ingested bacteria; when this acid is secreted in excess, it causes acidity and indigestion. An antacid relieves this through a neutralization reaction — acid + base → salt + water — using a mild, safe base that raises the stomach's pH back toward normal without damaging the stomach lining.

Application: Here, X is hydrochloric acid, the digestive acid the stomach secretes, and Y is milk of magnesia (a suspension of magnesium hydroxide) — a mild base commonly used as an antacid to neutralize excess stomach acid and relieve indigestion. This pairing matches both roles described in the stem.

Cross-check — why the other pairings fail:

• Sodium carbonate is a strong industrial alkali (washing soda, used in glass-making) — too harsh for the stomach lining, so it is not the therapeutic antacid used to treat indigestion.

• Swapping the roles — treating the base as the stomach's secretion and the acid as the remedy — reverses the chemistry: the stomach does not secrete a base for digestion, and dosing more acid into an already over-acidic stomach would worsen indigestion, not relieve it.

• Ascorbic acid (vitamin C) is itself an acid, so it cannot neutralize excess stomach acid — it would not relieve indigestion.

So X = hydrochloric acid and Y = milk of magnesia is the pairing that fits both the stomach's digestive chemistry and the neutralization principle used to treat acidity.