Magoosh Official Explanation:
Split #1: verb tense. The rise in cases of this disease is something that happened in the past, and may still be happening—that's unclear. Either the simple past "fueled" or present perfect "has fueled" could be correct—choices (A) & (B) & (D) & (E) have these. The past perfect would be correctly only if we wanted to establish contrast with another past event, but that doesn't make sense in this context: it changes the meaning too much. The past perfect "had fueled" is incorrect—choice (C) makes this mistake.
Also, the progressive tense in (B) is awkward: it's almost redundant to use the progressive tense to say that something "is progressing." Choice (B) is wrong.
Split #2: idiom with "fear."
When we are afraid of something happening, we need to use that: "the fear that X will do this."
When we are afraid of a single noun, we use of: "fear of flying", "fear of spiders", etc.
When we are afraid on behalf of something for which we have concern, then we use for: "I fear for my life", "I fear for the future of their marriage", etc.
Here, what is feared is the "full blown epidemic" and the fact that it may already be underway. Here, the "for" construction is entirely incorrect: choices (A) & (C) make this mistake and are incorrect. The "of" construction in (E) is acceptable. The other two choices have "that" clauses, which are correct.
These two splits leave us with (D) and (E). Choice (D) is sleek, elegant, and powerfully direct. Choice (E) is a dismally colloquial phrasing that does not stand up to logical analysis and would not bear examination in any respectable source of writing. Choice (E) is a complete disaster and is incorrect. Choice (D) is the best answer.