Official Solution:Due to the highly sensitive nature of their work, all State Department staff members were instructed to store their files on secure servers. A. Due to the highly sensitive nature of their work, all State Department staff members were instructed to store their files on secure servers.
B. Because diplomatic work was highly sensitive, the State Department’s staff had been instructed to store all files on secure servers.
C. Whereas diplomatic work was highly sensitive, the State Department’s staff was instructed to store their files on secure servers.
D. Because of the highly sensitive nature of their work, all State Department staff members were instructed to store their files on secure servers.
E. The State Department’s staff had been instructed to store their files on secure servers, since their work was highly sensitive.
This looks like a nice, clear, straightforward, mechanical question, with lots of nice pronoun and verb tense and modifier issues. I like it.
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(A) Due to the highly sensitive nature of their work, all State Department staff members were instructed to store their files on secure servers.
Whenever you see "due to", you'll want to ask: what does the phrase "due to _______" modify? In this sentence, "due to the highly sensitive nature of their work" is trying to describe "were instructed" -- but that doesn't work, since "due to _____" can only modify a noun. And it definitely doesn't make sense to say that "all State Department staff members... were due to the highly sensitive nature of their work." Eliminate (A).
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(B) Because diplomatic work was highly sensitive, the State Department’s staff had been instructed to store all files on secure servers.
The verb tenses don't make any sense in (B). Whenever you see "had + verb" (past perfect tense, if you like jargon), the action has to occur before some other time marker in the past (usually, another action in simple past tense). In this case, the sentence seems to be saying that the staff had been instructed to use secure servers first, and then the diplomatic work became highly sensitive later. That doesn't make sense, so we can get rid of (B).
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(C) Whereas diplomatic work was highly sensitive, the State Department’s staff was instructed to store their files on secure servers.
"Staff" is singular, but "their" is plural, so (C) can be eliminated easily enough.
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(D) Because of the highly sensitive nature of their work, all State Department staff members were instructed to store their files on secure servers.
This looks fine to me. Pronoun agreement is fine, and we don't have any issues with "due to" or verb tenses. We'll keep this one.
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(E) The State Department’s staff had been instructed to store their files on secure servers, since their work was highly sensitive.
(E) gives us nicer-sounding versions of the errors in (B) and (C).
Answer: D