Which property describes thermosetting plastics?

Thermosetting plastics are characterized by their ability to harden when heat is applied and form a rigid structure that cannot be reshaped afterward. This process involves a chemical change, meaning that once they are cured, they cannot be remelted or reformed by applying heat. This property makes them ideal for applications requiring durability and resistance to heat and chemicals, such as in electrical insulations, adhesives, and various composite materials.

The other options describe different types of materials or properties. For example, some plastics can be reprocessed multiple times when heated, indicative of thermoplastic behavior rather than thermosetting. Flexibility is also a property of certain types of materials that do not apply universally to all thermosetting plastics, which can be quite rigid. Lastly, the ability to melt and reform is a defining characteristic of thermoplastics rather than thermosetting plastics.