Final answer:
H₂ is the compound that is a molecule, as it is a diatomic molecule consisting of two hydrogen atoms bonded covalently.
Step-by-step explanation:
Among the listed compounds, H₂ is a molecule. A molecule is defined as a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction. In the case of H₂, it consists of two hydrogen atoms bonded covalently, forming a diatomic molecule, which is a pure chemical substance. In contrast, compounds like NaCl, MgBr₂, and AlCl₃ are all ionic in nature, with their constituents being ions rather than molecules.