Could this also mean "which daughter-in-law are you"?
Yes, it's either. From KBBI: menantu/me·nan·tu/ n istri atau suami dari anak kita
I think the sentence sounds fine without the word 'one' in it.
Which one is your son-in-law? = Yang mana satu menantu kamu? or Yang mana satukah menantu kamu?
The "Correct" answer also had the numeral "1" instead of the word "one" which is not at all natural.
What does "mana" mean here?
"Yang mana" together means "Which", rather than "Where"
Is it not better to use "who" when referring to a person: Siapa memantu kamu?
Why is "child-in-law" marked incorrect???
DL is pretty inconsistent.