Hi, I will try to "enlighten" this:
First in A) against their struggle is redundant, opposition is always against something or somebody

You cannot omit “did” here, you can omit "encountered" but you still need the auxiliary verb here.

420. In the United States, trade unions encountered far more intense opposition against their struggle for social legitimacy than the organized labor movements of most other democratic nations.
(A) against their struggle for social legitimacy than
(B) in their struggle for social legitimacy than did
(C) against their struggle for social legitimacy as
(D) in their struggle for social legitimacy as did
(E) when they struggled for social legitimacy than has

B is the correct answer...A is wrong since it omits the important word "did" which helps to draw correct parallelism between the 2 parts of the sentence

if A, means:
trade unions encountered far more intense opposition than the organized labor movements

comparison objectives are intense opposition and the organized labor movements

the logic is wrong


B means:
trade unions encountered far more intense opposition than the organized labor movements did

comparison objectives are trade unions andthe organized labor movements

this is correct

First and foremost guyzzz!!! cancel out the options that use the word 'against', as it cannot be used with struggle. It is wordy.. Already struggle itself indicates that there is a contrast or dispute going on with the word opposition. It becomes wordy

Then you are left with options B and D, D cancels out for using the wrong modifier more - as..
E is wordier very awkard

Hence B....