Since 1966, roughly 21 percent of those enlisted in the USMC

Dear nahid78 & fleamkt,

I'm happy to respond.

My friends, we have a tricky issue here. This is a poor question, and it was written by somebody who simply doesn't understand the GMAT SC. Thus, the OA provided by the author of the question is wrong by GMAT standards. The best answer is (B), as others on this thread have argued, but unfortunately, I do not have the right to change the OA. You see, the question does not belong to me. If it were a Magoosh question with a mistake, I would have ever right to change the OA. Also, if the person posting the question had made a mistake copying from the source, I would have the right to change the question. But when the author of the question is the one who makes the mistake, that person is the owner of the question, and I don't have the legal right to alter it.

Here's the question.
Since 1966, roughly 21 percent of those enlisted in the USMC had reached an E-5 ranking in fewer than three years.
(A) had reached an E-5 ranking in fewer than three years
(B) have reached an E-5 ranking in less than three years
(C) have reached an E-5 ranking in under three years
(D) had reached an E-5 ranking in less than three years
(E) have reached an E-5 ranking in fewer than three years

Split #1: "fewer" vs. "less"
We use "fewer" for countable items (e.g. cars, countries, situations, businesses, etc.)
We use "less" for things that come in continuous bulk (time, space, money, etc.)
The case that always confuses people, and that confused the author of this question, is the case involving units. When we have units of quantities that come in uncountable bulk, we do NOT treat the units as countable entities. Thus
less than three years = less time than three years
less than five miles = less distance that five miles
less than $12 = less money than $12
All of those are correct. It would be incorrect to use "fewer," as the question author did. Choice (A) & (E) are wrong by GMAT standards.

Split #2: present perfect vs. past perfect
We use the past perfect tense when we need to show that one past action happened before another action. The past perfect is simply wrong here. We use the present perfect tense to show
(a) when an action began in the past and is still continuing, or
(b) when an action took place in the past, started & finished in the past, but its effects are still felt in the present time.
The present perfect tense is ideal here, because when a soldier is promoted to a rank, the action of the promotion is a one-time event, but the effect, the new rank, is something that continues into the present.
Choices (D) & (E) are wrong.

The "under three years" in (C) is a bit casual. This is not wrong enough to be a wrong answer by GMAT standards.

The strongest answer is (B).

Does all this make sense?
Mike