Nitrogen is liberated by the thermal decomposition of only-

(A)NH4NO2

(B)NaN3

(C)(NH4)2Cr2O7

(D)All of these

The correct option is (D) All of these.

Thermal decomposition refers to a chemical reaction in which a compound is broken down into simpler substances by the application of heat. In the case of the compounds mentioned:

(A) NH4NO2 (Ammonium nitrite) decomposes into N2 (nitrogen gas) and H2O (water vapor).
2NH4NO2 → 2N2 + 4H2O

(B) NaN3 (Sodium azide) decomposes into N2 and Na.
2NaN3 → 3N2 + 2Na

(C) (NH4)2Cr2O7 (Ammonium dichromate) decomposes into N2, Cr2O3 (chromium(III) oxide), and 4H2O.
(NH4)2Cr2O7 → Cr2O3 + N2 + 4H2O

Therefore, all three compounds undergo thermal decomposition and liberate nitrogen gas (N2).