Ever since cornelius was hypnotized, he has been flapping his arms whenever he hears anyone say, "hello." cornelius's arm-flapping most likely results from

I believe the answer is: a posthypnotic suggestion.

a posthypnotic suggestion refers to the event when people who are being put under a hypnosis are unable to recall the things that happen to them under the  hypnotic influence. The erased memory usually could only come back after receiving a certain trigger (sound or a word)

answer; participant observation method;

answer; education;