What characteristic defines thermoplastics when subjected to heat?

Thermoplastics are defined by their ability to soften when heated and return to a solid state upon cooling. When subjected to heat, thermoplastics become pliable and can flow like a liquid, which allows them to be easily molded into various shapes. This property makes thermoplastics highly versatile for manufacturing processes such as injection molding and extrusion.

In contrast, the other options either describe behaviors not associated with thermoplastics or apply to different types of materials. For instance, thermosetting plastics, distinct from thermoplastics, become hard and rigid upon curing, which is why they do not melt and cannot be remolded after the initial setting. Understanding the behavior of thermoplastics under heat is crucial for various engineering applications, including design, processing, and material selection.