OFFICIAL EXPLANATION


If the speaker took up cerebral exercise as a means to delay "senility" (loss of mental capacity as a result of old age), we can expect that the speaker's mental acuity (sharpness of mind) had begim to "degenerate" or "worsen." Atrophy means "a weakening or degeneration, especially through lack of use." Atrophy is particularly appropriate, then, if we consider the speaker's attempt to reverse this degeneration with exercise. Wane means "to decrease in vigor or power," so it functions in the same way that atrophy functions. Though (B) regress can mean "get worse," it implies a return to a previous state or previous behavior. Our speaker's condition seems progressive rather than regressive because he fears senility; in addition, there is no other choice that gives us a sentence with the same meaning. Choices $(A)$ and $(D)$ are correct.