Which characteristic defines exoskeletons?

Exoskeletons are indeed defined by their nature as hard, protective structures located on the outside of an organism's body. This distinction is crucial because exoskeletons serve multiple important functions for the organisms that possess them, such as providing a sturdy shield against environmental hazards, predators, and physical damage.

In addition to protection, exoskeletons also play a role in facilitating movement. They provide a surface for muscle attachment, enabling the muscles to pull against the exoskeleton to create movement. This combination of protective and structural benefits is a key characteristic of exoskeletons found in many arthropods, such as insects and crustaceans.

The other options reference characteristics that do not align with the defining features of exoskeletons. For example, internal structures are indicative of an endoskeleton, while flexibility and softness typically describe different biological structures, such as skin or membranes rather than exoskeletal materials, which are generally rigid and strong.