Which characteristic is associated with thermoplastic materials?

Thermoplastic materials are specifically characterized by their ability to soften upon heating and harden upon cooling. This property allows them to be reshaped and reformed multiple times without undergoing any chemical change. When heated, the molecular structure of thermoplastics becomes more flexible, enabling them to be molded into various shapes. Upon cooling, they solidify again, retaining the new shape. This unique behavior makes them highly versatile for a variety of applications, including roofing materials, where they can be easily installed and adjusted.

The other characteristics listed do not apply to thermoplastics: solidifying irreversibly when heated describes thermosetting plastics, maintaining rigidity at high temperatures is not a feature of thermoplastics but rather a property of certain composites, and while easy installation can be a benefit of thermoplastics, it is not a defining characteristic of the material itself.