Question:
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

Source: MGMAT SC Question Bank



a) that were hidden during the Nazi occupation of the Netherlands by a gentile Dutch couple, though they were eventually discovered

b)that were hidden by a gentile Dutch couple during the Nazi occupation of the Netherlands, though they were eventually discovered

c) whom a gentile Dutch couple hid during the Nazi occupation of the Netherlands but were eventually discovered

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

I did this problem yesterday and a little confused by part of the explanation provided by MGMAT. Specifically "The original sentence uses the relative pronoun "that" where "who" is preferred because the antecedent is a group of people"

Can someone please elaborate on this point, I was definitely not aware of this. Thanks