Which of the following describes the material known as a polymer?

Polymers are large molecules composed of repeating structural units, typically connected by covalent chemical bonds. The characteristic that most accurately describes polymers is their sturdy and durable nature, which is commonly associated with materials that resemble plastic. Many polymers exhibit qualities such as strength and stability, making them useful in a wide range of applications, from everyday items to specialized industrial products.

While polymers can indeed be flexible and elastic, the defining feature of many is their robustness, suitability for various environments, and ability to maintain utility over time. This durability often leads to the polymer being used as a material that will withstand stress without deforming easily, making it particularly advantageous in many applications.

The other options describe characteristics that may apply to specific types of materials but do not universally define polymers as a class. For instance, while some polymers can be low-density and lightweight, not all are, and the same goes for flexibility and fragility. Thus, the emphasis on sturdiness and durability resonates best with the general description of polymers.