Which one of the following is not a colourless compound?

Total number of acidic oxides among $N_2O_3, NO_2, N_2O, Cl_2O_7, SO_2, CO, CaO, Na_2O$ and $NO$ is $...........$

Inertness of $N_2$ gas is due to

The incorrect statement regarding $15^{th}$ group hydrides $(EH_3)$ where $E = N, P, As, Sb, Bi$ is:

$A$ metal $(X)$ dissolves in both dilute $HCl$ and dilute $NaOH$ to liberate $H_2$ gas. Addition of $NH_4Cl$ and excess $NH_4OH$ to an $HCl$ solution of $(X)$ produces $(Y)$ as a precipitate. $(Y)$ is also produced by adding $NH_4Cl$ to the $NaOH$ solution of $(X)$. The species $(X)$ and $(Y)$,respectively,are:

$NH_3 + O_2 \xrightarrow{\text{Catalyst}} NO + H_2O$. This step is used in which process?