Final answer:
Wisdom teeth (third molars) in humans are an example of a vestigial feature, which were useful to our ancestors for effective grinding of rough plant food but lost their functionality due to changes in diet and jaw size.
Step-by-step explanation:
In the context of biology, vestigial features are physical characteristics that were useful in an organism's evolutionary past, but are no longer functional or have decreased functionality in present times. In humans, one prime example of a vestigial feature is wisdom teeth.
Wisdom teeth, or third molars, are believed to have been useful to our ancestors who had a diet consisting of more rough plants and needed extra molars for effective grinding. But with changes in the human diet and jaw size over time, wisdom teeth no longer have such significance, often leading to complications due to lack of space and making them a clear case of a vestigial characteristic. Therefore, among the options provided: a. wisdom teeth, b. toes, c. pelvic girdle, and d. humerus, 'a. wisdom teeth' is the correct answer.
Learn more about vestigial features