In Duma’s method, the gas which is collected in Nitrometer is

In addition to carbon dioxide and water, free nitrogen is produced when a nitrogen organic molecule is burned with too much copper oxide in an environment of carbon dioxide. 
Chemical processes include: 
$$\begin{gathered} \mathrm{C}+2\mathrm{CuO} \to {\mathrm{CO}}_2+2\mathrm{Cu} \\ 2\mathrm{H}+\mathrm{CuO} \to {\mathrm{H}}_2\mathrm{O}+\mathrm{Cu} \end{gathered}$$

Similarly,
$\mathrm{Nitrogen}+\mathrm{CuO} \to {\mathrm{N}}_2+\mathrm{Oxides} \mathrm{of} \mathrm{nitrogen}$, thus nitrometer will contain nitrogen.
$\mathrm{Oxides} \mathrm{of} \mathrm{nitrogen}+\mathrm{Cu} \to {\mathrm{N}}_2+\mathrm{CuO}$
The mixture of gases is collected over $\mathrm{KOH}$ solution that absorbs ${\mathrm{CO}}_2$​.
Thus, the correct answer is option (A) ${\mathrm{N}}_2$.