Translation:Which one is your son-in-law?
I think the sentence sounds fine without the word 'one' in it.
The "Correct" answer also had the numeral "1" instead of the word "one" which is not at all natural.
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