Several species of fish in the genus astyanax occur only in the waters in completely dark underground caves. these blind cave fish have no external eyes, but have very tiny and functionless eyes under the skin. genetic studies clearly indicate that the blind cave fish are evolved from surface-dwelling species with typical and functional eyes. the eyes of these blind cave fish are, therefore, an example of

a vestigial structure

Explanation:

Vestigial structures are a rudimentary (or even functionless) version of a body part, but they have important functions in a closely correlated or evolutionarily close species, an example of which is the presence of eyes in fish of the genus Astyanax. The existence of these vestigial structures is strong evidence that evolution occurs in organisms, since this structure, today without much apparent function, may in the past have been extremely important to the ancestors of that species.

A vestigial structure

Explanation:

Several species of fish in the genus Astyanax occur only in the waters in completely dark underground caves. These blind cave fish have no external eyes, but have very tiny and functionless eyes under the skin. Genetic studies clearly indicate that the blind cave fish are evolved from surface-dwelling species with typical and functional eyes. The eyes of these blind cave fish are, therefore, an example of a vestigial structure.

the answer is c) an oak tree