"They don't need their socks."
Translation:הם לא צריכים את הגרביים שלהם.
Both feminine and masculine are accepted:
- (fem.) הן לא צריכות את הגרביים שלהן
- (masc.) הם לא צריכים את הגרביים שלהם
rokssolana made an interesting point that the subject pronoun and possessive pronoun don't necessarily have to match. However, at this point in time, Duo seems to only accept them as a matching set:
הן + שלהן -OR- הם + שלהם
In this sentence, the multiple choice exercise marks the option "הם לא צריכים את הגרביים שלהן" as incorrect, although it is technically correct (שלהן is also "their"), even if it would mean that they do not need someone else's socks, the socks of some women or girls, not their own. But it is still translated as "They don't need their socks", so it should be a correct option.
Maybe your masculine form wasn't accepted because what you wrote wasn't actually correct. I had that happen to me, and I thought at first it was a gender issue, but no - I'd just screwed up my masculine form of the sentence (forgot את - classic). The app doesn't fix your sentence, it just displays the correct feminine form of the answer.
Can you elaborate more? Because the sentence above is masculine. In theory, there are two possible translations.
הם לא צריכים את הגרביים שלהם.
הן לא צריכות את הגרביים שלהן.
Yes, socks are feminine, but there is nothing that relates to the gender of the socks in the sentence. What determines the gender is the subject - הם or הן.