Final answer:
Vestigial structures are remnants of an organism's evolutionary past which no longer serve their original function, such as the human appendix or whale pelvic bones. These structures provide evidence for evolution and common ancestry, showing that species evolve over time and may retain nonfunctional or minimally functional anatomical features.
Step-by-step explanation:
The piece of evidence that refers to structures which most likely had a function at one time, but no longer serve any known function, is B) Vestigial structures.
Vestigial structures are anatomical remnants that were important in the organism's ancestors but are no longer used in the same way. Some vestigial structures may retain lesser or changed functions, while others are merely residual. Examples of vestigial structures include the human appendix, the pelvic bones in whales, and the wings on flightless birds. The presence of such structures provides evidence for evolution, suggesting that organisms have evolved from ancestors in which these structures were functional. This supports the theory that different species share common ancestry and have diverged over time as their environment and way of life have changed.
Vestigial structures shrink in size over generations because a full-sized, unused structure can reduce an organism's fitness by being energetically costly to grow and maintain. Thus, natural selection may favor organisms with smaller, less costly versions of these structures.