An "eagle" is an example of an "airborne" (in the air, flying) creature. So, to solve this analogy you need to determine which of these is a "subterranean" creature. Well, to solve this you obviously need to know what "subterranean" means. There are two relevant roots in the word to consider: First, sub- means below, beneath, under and second, terra- means Earth. So, "subterranean" means below the Earth. Which of these creatures lives "below the Earth?" A "worm."