I would be happy to help with that question.
1. The substances that lack a definite heat of fusion are categorized as amorphous solids.
2. Amorphous solids do not have a specific melting point, unlike crystalline solids that have a distinct heat of fusion associated with their melting process.
3. Examples of amorphous solids include glass, plastic, and some types of rubber. These substances have a disordered molecular structure, which is why they do not exhibit a sharp melting point like crystalline solids.
Therefore, in the context of the question, the correct answer is:
(C) amorphous solids.
