This sentence is fairly complicated, but if you recognize that it is built around a comparison, the question doesn't have to be very difficult. To simplify: "Unlike the brain of a chickadee that lives in the wild..., a caged chickadee...." What follows the comma must be comparable to "the brain" of one type of chickadee; it must be the brain of another sort of chickadee. We can't compare brains to chickadees.

Check all the choices for what follows the comma. (A) is wrong because it compares "the brain" to "a caged chickadee." (C) is wrong, comparing "the brain" to "far fewer neurons." (D) and (E) make a similar mistake, comparing "the brain" to "the production of new neurons." The only properly drawn comparison is found in choice (B).