Answer:
I Dont think it would be D. Breeding with other species is possible, but extremely rare. I Think its most likely C, because C shows an adaptation, their beak changing in size. They might be changing so that their beaks are strong enough to break through plants or ground and drink the water from it. There might also be other reasons, that is just my prediction. If it gets too harsh, however, there might not be a way to adapt to it and they may go extinct. But as i said i think it would be C.