When MnO₂ and H₂SO₄ is added to a salt (A), the greenish yellow gas liberated as salt (A) is :

The correct option is NH₄Cl.

When manganese dioxide (MnO₂) and concentrated sulfuric acid (H₂SO₄) are added to a chloride salt like ammonium chloride (NH₄Cl), chlorine gas (Cl₂) is liberated. Chlorine is a characteristic greenish-yellow gas with a suffocating smell.

Let's understand the step-by-step reactions that take place:

1. The ammonium chloride salt reacts with sulfuric acid to produce hydrogen chloride (HCl) gas:
NH₄Cl + H₂SO₄ → NH₄HSO₄ + HCl

2. The manganese dioxide (MnO₂), which is a strong oxidizing agent, then oxidizes the hydrogen chloride (HCl) to release chlorine gas:
MnO₂ + 4 HCl → MnCl₂ + 2 H₂O + Cl₂ ↑ (greenish-yellow gas)

3. The combined overall chemical reaction is:
MnO₂ + 2 H₂SO₄ + 2 NH₄Cl → MnSO₄ + (NH₄)₂SO₄ + 2 H₂O + Cl₂ ↑

Let's analyze the behavior of the other options under similar conditions to verify:
• NaBr: Reaction of a bromide salt with MnO₂ and H₂SO₄ liberates bromine (Br₂) gas, which is a reddish-brown vapor.
• CaI₂: Reaction of an iodide salt with MnO₂ and H₂SO₄ liberates iodine (I₂) gas, which is a violet vapor.
• KNO₃: Reaction of a nitrate salt with H₂SO₄ yields nitrogen dioxide (NO₂) gas, which is also a reddish-brown gas.

Consequently, only the chloride salt, NH₄Cl, yields the characteristic greenish-yellow chlorine gas.