The Diary of Anne Frank tells the true story of a young girl and her family that were hidden during the Nazi occupation of the Netherlands by a gentile Dutch couple, though they were eventually discovered.
"¢ that were hidden during the Nazi occupation of the Netherlands by a gentile Dutch couple, though they were eventually discovered
"¢ that were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands, though they were eventually discovered
"¢ whom a gentile Dutch couple hid during the Nazi occupation of the Netherlands but were eventually discovered
"¢ who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands but were eventually discovered
"¢ who were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands even though they were eventually discovered

A- that refers to things not people (though logically it refers to people as they were hidden by dutch couple)
B- same as A
Now among C,D, E
C- whom is an object , we need a subject for the verb 'were eventually discovered'
For the but the part the sentence it would be :
whom.........were eventually discovered - which doesnot make sense with object (whom, we need who)
D, E
E- even though means both happened at same time but the as per the logic of the sentence they hid first and later they were discovered. So E is wrong

Answer should be D

Who correctly refers to the family and the young girl, followed by but were eventually discovered.It also refers to the plural formed by using and.

Whom does not correctly modify the preceding subjects

Eg: To whom are you giving this ring?
Ans: Mary

E.g: The Browns, who are my neighbors, gave me the flowers.