Final answer:
Molecular solids have the lowest melting points.
Step-by-step explanation:
The substances with the lowest melting points are molecular solids. Molecular solids are composed of individual molecules held together by weak intermolecular forces. These forces are much weaker than the ionic or metallic bonds found in ionic and metallic solids, respectively. As a result, molecular solids have lower melting points compared to ionic and metallic solids.