OE


The sentence indicates that the number of PhD applicants in linguistics "has either grown or held steady in each of the past 15 years" while also suggesting that this fact is surprising. This could be surprising for a number of reasons, but the only words that would pair in this sentence are "moribund," which means dying, and "waning," which means growing smaller. "Waxing" and "burgeoning" do form a pair, but if the field were growing, it would not be a surprise that the number of applicants has also grown. Neither "dissolute" nor "debased," which both mean lacking in morals, fits the desired meaning of the blank.