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

Step 1: Understand the Reagents and the Product

The test involves reacting a salt with Manganese dioxide ($\ce{MnO2}$) and Concentrated sulphuric acid ($\ce{H2SO4}$). A greenish-yellow gas is liberated.

Step 2: Correlate properties to the specific gas

Each gas common to the Conc. $\ce{H2SO4}$ testing group has a defining color/smell:

• Nitrate salts yield brown nitrogen dioxide ($\ce{NO2}$) vapor.
• Bromide salts yield reddish-brown bromine ($\ce{Br2}$) vapor.
• Iodide salts yield violet iodine ($\ce{I2}$) vapor.
• Chloride salts specifically yield greenish-yellow chlorine ($\ce{Cl2}$) gas when heated under strong oxidizing mixtures (like $\ce{MnO2} + \ce{H2SO4}$).

Step 3: Analyze the reaction mechanics

The salt (A) containing chloride ions reacts first with $\ce{H2SO4}$ to release hydrogen chloride ($\ce{HCl}$) gas. Then, the powerful oxidizing reagent $\ce{MnO2}$ oxidizes the $\ce{HCl}$ entirely into elemental chlorine.

$\ce{2NaCl + H2SO4 -> Na2SO4 + 2HCl}$ $\ce{MnO2 + 4HCl -> MnCl2 + Cl2 ^ + 2H2O}$ The liberated $\ce{Cl2}$ gas is visibly greenish-yellow.

Step 4: Search for the chloride salt

Looking at the options, the only chloride-containing salt is Ammonium chloride ($\ce{NH4Cl}$).

Step 5: Conclusion

Therefore, salt A is $\ce{NH4Cl}$.

Key Points to Remember:

• $\ce{Cl2}$ gas = Greenish-yellow with a suffocating, pungent odor.
• $\ce{Br2}$ gas = Reddish-brown vapor.
• $\ce{I2}$ gas = Deep violet vapor.
• $\ce{MnO2} + \ce{H2SO4}$ is a very strong oxidizing combination used specifically to generate halogens and confirm the presence of halide ions.