The opening clause in this sentence is a modifier; what follows the comma must be what is modified. What is "hunted by poachers?" That's the Black Rhinoceros, so the Black Rhinoceros must follow the comma. (C), (D) or (E) must be correct.

In (C), the sentence should be complete even without the interpolated phrase "killed for...." However, "Black Rhinoceros and were reduced so heavily in number..." is not a complete sentence. (D) isn't the most attractive grammatical construction, but "were killed" and "were reduced" are parallel.

Choice (E) is wrong because the word "so" is misplaced. Compare "reduced so heavily in number that one of the subspecies was declared extinct" in (D) with "reduced heavily in number so that one of their subspecies..." in (E). The correct idiom to express the effects of a change like this one is "so...that," not "so that." Choice (D) is correct.