The decline of the mountain yellow-legged frog in the high reaches of the Sierra Nevada has become severe enough for the United States Fish and Wildlife Service quite possibly to list it as an endangered species in the near future.
Intended MeaningThe sentence is talking about two things. 1. The decline of frog. 2. United States Fish and Wildlife Service could enlist the frog as an endangered species. These two things happens independently and the first one is not deliberately causing the second.
(A) severe enough for the United States Fish and Wildlife Service quite possibly to list it
Incorrect : This option alters the intended meaning. The intended meaning is that the United States Fish and Wildlife Service could list the frog as endangered because of the decline in its numbers. This option implies that the frog intended to be severe enough to be listed
(B) severe enough so that the United States Fish and Wildlife Service could well list them
Incorrect : 1. Meaning issue 2. The pronoun "them" cant refer back to frog as frog is singular
(C) severe enough for it quite possibly to be listed by the United States Fish and Wildlife Service
Incorrect : Same meaning issue as in A
(D) so severe that the United States Fish and Wildlife Service could well list it - Correct. The pronoun it refers back to its logical antecedent "frog"
(E) so severe that they could well be listed by the United States Fish and Wildlife Service
Incorrect : The pronoun "they" cant refer back to frog as frog is singular