What term describes structures that serve no useful purpose in an organism?

Structures that serve no useful purpose in an organism are referred to as vestigial structures. These are remnants of evolutionary history that once had a significant function in the ancestors of the organism, but have since lost their original purpose. Examples of vestigial structures include the human appendix, which is a reduced version of a larger organ that assisted in digestion for herbivorous ancestors, or the pelvic bones in whales that are remnants from land-dwelling ancestors.

In contrast, functional structures refer to parts of an organism that have a direct and useful role in its survival or reproduction. Adaptive structures are those that have evolved to enhance an organism’s ability to survive in a specific environment. Essential structures are critical to an organism's life processes and overall function. Therefore, vestigial structures are specifically characterized by their lack of purpose in the current form of the organism, making this option the most accurate description.