I selected C.
Reasoning:
1. I feel, the use of past perfect is correct because the union acquiesced... before the United Mine Workers made a determined effort...
2. In GMAT, "where" is used for locations and never for human beings or a group of living beings.

I would have selected D.

Yes, the acquiescence happened before, but it is pretty clear from the sentence already, and I think therefore it does not need to be emphasized anymore.

I also think it is ok to use the prepostional construction in D, especially when the subject is a live animated thing, here humans (but works with animals too).

I agree with the "where" part and would not select A or B. So it is between C, D and E.

I have no idea why the OA is A. I am seeing so many wrong answers in so many texts (mostly in SC), it has become quite unreliable in my opinion. The best thing that works is to find some consensus in these forums, especially with the help of experts.

1st. had is correct bc next action follow
2nd While - incorrect bc changes the original meaning
3rd acquiesced to..... by - not for(B), in(C,D,E)
4th -acquiesced to its - not it(B)
5th -in support of(C)(even if appropriate) redundant - by supporting better

so A is left

Ok, maybe it is A after all. Let's just say that S0laris made me look into this problem deeper. I still do not agree with all his points, however. I have my test tomorrow, so I am on top of this one.

I started by looking at the difference between "where once" and "while once"

The bottom line is that "where" does not necessarily need to be a pronoun and thus refer to a location. I found the following explanation somewhat useful https://www.urch.com/forums/gmat-sentenc ... -once.html:

"where can be used in the sense of whereas, as can while. However, if you have to choose between while and whereas, you should go with whereas (or where in this case) if while can be ambiguous in the sentence, since it can mean whereas or at the same time that.

So, "while" here causes ambiguity. And so, it would be between A and B.

Secondly, I am very much aware with using "had" if an event takes place before another. But I also learned that if it is pretty clear that one event happens after another, you don't need to use "had". Now, there is one caveat, and maybe two. The first is: If the two events are not related, and the second, which I am assuming, is that if you need to choose between an answer that has "had" and one that does not have "had", but the one that does not have had makes another error, then still go with "had".

I don't think there is that relation and so I still do not think we need "had", however, there is a clear error in B with "it". So, I guess A is best after all.

So I sign off with this, in the hope that this is now pretty close.