Well...nice one. My pick is D

that Vs Who Vs Whom

in my opinion option D shows correct formation Parallelism


D. who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands but were eventually discovered

OA pls

IMO D

clearly that is wrong therefore A,B is out
E is out bcoz of wordiness

C and D are very close , my take is D as in C the two parts of sent separeted by but are not parallel in construction former is active and later is passive, which is corrected in D

wud appreciate if someone can differentiate

D. who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands but were eventually discovered
E. who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands even though they were eventually discovered

Tough one for me...
C is incorrect because parallelism is lost - whom...but were

Between D & E is a tough call and not because E is wordy - but because E changes the meaning of the sentence a lil bit. I liked the use of they in E though...
However I would go with D because it is grammatically correct.

A. that were hidden during the Nazi occupation of the Netherlands by a gentile Dutch couple, though they were eventually discovered [ pronoun ambiguity error. 'they' refers to whom??]

B. that were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands, though they were eventually discovered [same error as in A]

C. whom a gentile Dutch couple hid during the Nazi occupation of the Netherlands but were eventually discovered ['whom' is a pronoun used to refer to the object of the sentence. here 'the young girl and her family' are the subject of the sentence- were hidden during nazi operation. Hence it is the wrong use of pronoun]

D. who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands but were eventually discovered [correct! who correctly refers to the subject of the sentence]

E. who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands even though they were eventually discovered [same pronoun ambiguity error as in A]

Hope this helps!