What characteristic defines thermosets in comparison to thermoplastics?

Thermosets are defined by their characteristic ability to maintain their shape and structural integrity when exposed to heat. This is due to the cross-linked molecular structure formed during the curing process, which solidifies the material and prevents it from melting or reshaping upon subsequent heating. Unlike thermoplastics, which can be reheated and remolded multiple times without significant chemical change, thermosets undergo a chemical transformation that makes them rigid and heat-resistant. This unique feature makes thermosets suitable for applications that require materials to withstand high temperatures and mechanical stress without deforming.

The other characteristics listed do not accurately capture the essential nature of thermosets. For instance, thermoplastics can be melted and reshaped, which highlights their opposite characteristic. Therefore, the defining characteristic of thermosets is their ability to maintain their shape under heat, which aligns with the correct answer.