Nice SC I must say....I am writing my explantion, I might be wrong. I invite people for further discussion. But, here is my explanation

E and C uses HAD...so I am not very friendly with this use, and moreover, I can describe the situation better without HAD...SO dont prefer it here.

B and D has a problem, which is very interesting to note here...I feel these option has PARALLEL error....I can see use of Possessive pronoun in 2nd part of the sentence, but no such use in part 1. Hence, this makes these options faulty.

I am left with A.

Look at it this way:

A -- there is simply nothing wrong with this answer choice. But hold out and look at the others

B -- 'it' is not possessive and it should be. Wrong.

C-- 'While' is not correct, plus no need to change from 'Where' in the main example. In addition, 'the prejudices of its English-speaking members' is an unnecessarily lengthy way to say 'English-speaking member's prejudice'. Wrong.

D -- same thing with 'While'. Plus change from 'by supporting' to 'in supporting' muddies the meaning of the phrase! Wrong.

E -- same as C with the additional awkward construction of 'in its supporting' added. Just wrong.

Option A.
First of all, what does acquiesce means? To rest satisfied, to concur upon conviction, to assent to
So if that is the case the 'Union acquiesce by supporting' that parallelism it should follow directly.
Thu, option A is the right answer.

Option B.
'it' this is of no use here and does not really matter.Thus, this option is incorrect.

Option C.
'in support of' option A will be considered over this option because of the best use of words and idioms.
This option is incorrect.

Option D
This is similar to what happened to option d above the use of 'it' is highly awkward and does not make a useful meaning.

Option E.
'in it supporting of' this is sore to the eye and does not deserve any consideration.This option is incorrect.