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The correct answer is a diatomic molecule because it consists of two atoms of the same element bonded together. In this case, H₂ is composed of two hydrogen atoms. Diatomic molecules are common in nature and include elements like nitrogen (N₂), oxygen (O₂), and hydrogen (H₂), which exist in their elemental form as pairs of atoms.

The other options do not fit the definition of diatomic molecules. CO₂, or carbon dioxide, is a linear molecule made up of one carbon atom and two oxygen atoms; it is a triatomic molecule. NaCl, or sodium chloride, is an ionic compound formed from sodium and chloride ions, and does not contain diatomic molecules. CH₄, or methane, consists of one carbon atom bonded to four hydrogen atoms, making it a tetrahedral molecule, not diatomic.

Therefore, the only diatomic molecule listed is H₂.