The correct option is A Hydrogen chloride
Compounds are made up of two or more kinds of atoms, chemically combined, in a definite ratio. Here, hydrogen chloride (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 (O2), nitrogen (N2) and chlorine (Cl2) are made of only one type of atoms i.e., oxygen, nitrogen and chlorine respectively, hence they are classified as elements.