Answer: Option A
:
A
Compounds are made up of two or more kinds of atoms, chemically combined, in a definite ratio. Here, hydrogen chloride ($\text{HCl}$) is formed by chemical combination of hydrogen and chlorine in the ratio 1:1, hence it is a compound.
Elements are made up of only one kind of atoms. Since, oxygen $\left({\text{O}}_2\right)$, nitrogen $\left({\text{N}}_2\right)$ and chlorine $\left({\text{Cl}}_2\right)$ are made of only one type of atoms i.e., oxygen, nitrogen and chlorine respectively, hence they are classified as elements.