Which of the following is a primary characteristic of thermoplastic materials?

The primary characteristic of thermoplastic materials is that they can be softened repeatedly when heated. This property allows thermoplastic materials to be molded and remolded into different shapes without significant degradation to the material itself. When cooled, they harden again, maintaining the new shape. This feature is extremely advantageous in manufacturing and fabrication processes, as it provides versatility and efficiency in creating various products.

In contrast, the other characteristics listed do not apply to thermoplastic materials. For example, thermoplastics are known for their ability to be reshaped and reprocessed, which contradicts the notion that they cannot be reshaped. Thermoplastic materials also vary in flexibility and can be made both rigid or flexible, depending on their formulation, so saying they are inherently rigid and durable does not encompass their full range of properties. Additionally, while some thermoplastics may require specific molding techniques, the complexity is not a defining characteristic of all thermoplastics as many can also be formed through simpler processes.