On heating compound (A) gives a gas (B) which is constituent of air. This gas when treated with H2 in the presence of a…

Strategy: Find which compound (A) does NOT give a gas that is a constituent of air, which when treated with H₂ gives a basic gas.

Step 1: Analysing the Reaction Chain

• Gas B is "a constituent of air" — air contains $\ce{N2, O2, CO2, Ar}$, but most likely $\ce{N2}$ (major constituent).
• Gas C is "basic in nature" — treating $\ce{N2}$ with $\ce{H2}$ over a catalyst gives $\ce{NH3}$ (ammonia, basic). ✅

So: compound A → $\ce{N2}$ (B) → + $\ce{H2}$ → $\ce{NH3}$ (C)

Step 2: Compounds that Give N₂

• $\ce{NaN3}$ (Sodium azide): $\ce{2NaN3 ->[\ \Delta\ ] 2Na + 3N2}$ ✅ gives N₂
• $(\ce{NH4})_2\ce{Cr2O7}$: $\ce{(NH4)2Cr2O7 ->[\ \Delta\ ] N2 + Cr2O3 + 4H2O}$ ✅ gives N₂
• $\ce{NH4NO2}$: $\ce{NH4NO2 ->[\ \Delta\ ] N2 + 2H2O}$ ✅ gives N₂
• $\ce{Pb(NO3)2}$: $\ce{2Pb(NO3)2 ->[\ \Delta\ ] 2PbO + 4NO2 + O2}$ — gives $\ce{NO2}$ and $\ce{O2}$, not $\ce{N2}$ ❌

Step 3: Answer $\ce{Pb(NO3)2}$ does not give $\ce{N2}$ on heating. So (A) should NOT be $\ce{Pb(NO3)2}$.

$\boxed{\text{Answer: (B) }\ce{Pb(NO3)2}}$