Which feature differentiates covalent molecular substances from ionic compounds?

Covalent molecular substances are characterized by the presence of molecules held together by covalent bonds, which involve the sharing of electrons between nonmetals. Ionic compounds, on the other hand, consist of charged ions that are held together by ionic bonds formed through the electrostatic attraction between positively and negatively charged ions.

The correct choice highlights that covalent molecular substances typically have lower melting points compared to ionic compounds, which are generally solid at room temperature and possess high melting points due to the strong ionic bonds. Covalent substances may also exhibit varying intermolecular forces, like van der Waals forces or hydrogen bonds, which can be relatively weaker than the ionic bonds found in ionic compounds.

The other features, while relevant to assessing properties of substances, do not specifically distinguish between covalent molecular substances and ionic compounds in the same way. For instance, the presence of charged ions is a defining feature of ionic compounds, and the ability to conduct electricity is more closely related to the mobility of ions in a liquid state, which again is characteristic of ionic compounds rather than covalent substances. Likewise, while covalent and ionic compounds can exist in solid states, it's the melting point and the intermolecular force strength that truly provide a clear distinction between the two types of compounds.