Which of the following describes a structural adaptation?

A structural adaptation refers specifically to physical characteristics of an organism that enhance its ability to survive in its environment. This can include features such as the shape of a bird's beak, the thickness of a mammal's fur, or the coloration of an insect, all of which serve distinct functions that aid in survival, reproduction, or the acquisition of resources.

For example, the long neck of a giraffe is a structural adaptation that allows it to reach high leaves that other herbivores cannot access, providing it with a food source that others cannot exploit. The physical nature of this adaptation is crucial; it is not about behavior or internal processes, but rather about the organism's tangible characteristics that directly affect its interaction with the environment.

The other options refer to non-structural adaptations. Learned behaviors focus on acquired skills rather than inherent physical traits. Physiological changes relate to internal functions and reactions, which are distinct from structural adaptations. Changes in reproductive habits may influence survival and success but do not involve physical structures, making them behavioral adaptations rather than structural ones.