Which oxide of nitrogen is a coloured gas?

Correct statements from the following are: $A$. Nitrogen in oxidation states from $+1$ to $+4$ disproportionates in acid medium. $B$. Nitrogen has the ability to form $d\text{--}p$ multiple bonds with itself and other elements with small size and high electronegativity. $C$. $N$-$N$ single bond is stronger than $P$-$P$ single bond. $D$. Nitrogen has highest density in its group due to small size. $E$. The maximum covalency of nitrogen is four since it has only four valence orbitals for bonding.

Ammonia reacts with an excess of chlorine to form:

In the Haber process,hydrogen is obtained by reacting methane with steam in the presence of a $NiO$ catalyst. This process is known as steam reforming. Why is it necessary to remove $CO$ when ammonia is obtained by the Haber process?

Which of the following elements does not exhibit the inert pair effect?

Which oxide is least acidic?