Preparation of Extra Pure Nitrogen Gas

To determine the method for obtaining extra pure (ultra-pure) nitrogen gas ($\text{N}_2$), let us analyze the reactions involved in each option upon heating:

1. Reaction of $\text{NH}_3$ with $\text{CuO}$:
   $2\text{NH}_3 + 3\text{CuO} \xrightarrow{\Delta} 3\text{Cu} + \text{N}_2 + 3\text{H}_2\text{O}$
   This reaction produces nitrogen gas, but it is contaminated with water vapor and unreacted ammonia.

2. Thermal decomposition of $\text{NH}_4\text{NO}_3$:
   $\text{NH}_4\text{NO}_3 \xrightarrow{\Delta} \text{N}_2\text{O} + 2\text{H}_2\text{O}$
   Heating ammonium nitrate produces dinitrogen oxide (nitrous oxide, $\text{N}_2\text{O}$) and water vapor, not nitrogen gas ($\text{N}_2$).

3. Thermal decomposition of $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$:
   $(\text{NH}_4)_2\text{Cr}_2\text{O}_7 \xrightarrow{\Delta} \text{Cr}_2\text{O}_3 + \text{N}_2 + 4\text{H}_2\text{O}$
   This is a standard laboratory method for preparing nitrogen gas. However, the gas produced contains moisture as well as trace impurities of oxides of nitrogen ($\text{NO}$ and $\text{NO}_2$).

4. Thermal decomposition of Barium Azide $\text{Ba(N}_3)_2$:
   $\text{Ba(N}_3)_2 \xrightarrow{\Delta} \text{Ba(s)} + 3\text{N}_2\text{(g)}$
   Pure metal azides, such as barium azide $\text{Ba(N}_3)_2$ or sodium azide $\text{NaN}_3$, undergo thermal decomposition to yield the corresponding metal and nitrogen gas. Since barium is a non-volatile solid, nitrogen gas is the only gaseous product obtained, resulting in extra pure (ultra-pure) $\text{N}_2$ free from oxygen, moisture, and nitrogen oxides.

Thus, extra pure $\text{N}_2$ can be obtained by heating $\text{Ba(N}_3)_2$.

Correct Option: (D)