Final answer:
The appendix is the correct answer for a vestigial structure, as it is a small, evolutionary remnant of an organ that once had a function in digestion but is no longer necessary in humans.
Step-by-step explanation:
The correct choice for a vestigial structure is a) appendix. Vestigial structures are parts of the body that have become reduced in size over time because they no longer serve the function they used to in our ancestors. The appendix is an example of this, as it is a small remnant of a once-larger organ used for digesting food that is not necessary in humans today. The vestigial structures can decrease in size over evolutionary time because there is no longer a selective pressure to maintain them, and a full-sized, unused structure might reduce an organism's fitness by wasting resources.
Vestigial structures are structures that have reduced in size because they are no longer needed in evolution. The appendix is an example of a vestigial structure. It was once used for digesting food but now serves no purpose in humans. Therefore, the choice that represents a vestigial structure is a) appendix.