The correct answer among the choices provided is the third option. The statement that is least supportive of the main idea is "Barnacles can also be found on boats." In this sentence, barnacles and boats are the keywords. These keywords are not related to the main idea which is the tidal pools.
I believe the answer is C. Symbolism, representing a voice being trapped in a dark place.
A mimic echo of a voice, being trapped in a shell, a dark place.
Hope this helps. - M