A compound is a substance made up of two or more different elements that are chemically bonded together.
Among the options you provided:
A. Nitrogen (N2) - This is a diatomic molecule made of two nitrogen atoms; it is not a compound because it consists of only one element.
B. Hydrogen (H2) - This is a diatomic molecule made of two hydrogen atoms; it is also not a compound as it consists of only one element.
C. Helium (He) - This is a noble gas and consists of single, unbonded helium atoms; it is not a compound.
D. Hydrochloric acid (HCl) - This is a compound that consists of hydrogen (H) and chlorine (Cl) atoms chemically bonded together.
Therefore, the correct answer is D. Hydrochloric acid (HCl).