She's a daughter is not the answer, and neither is she is a daughter. Word-by-word this is the translation: उनकी or o-na-ki, means "They", एक or ek, means "one", एक or bae-ti, means daughter, है or hai, means have, so it would translate to "They have a daughter."(as it says above) taking into account Hindi grammar rules.
From what I learned उनकी is a possessive pronoun. But "they" is just a pronoun - so why using उनकी here?
It's just how possessives are formed in Hindi. The word-for-word translation (which isn't what Duolingo's looking for) would be something like 'Their one daughter is'. For possession of objects you use 'ke pas' - literally something like 'near to me'.
How is this "they" differing from other words for they, like वे? How do we know which one to use?
So when saying thier or unake you don't need to use possessive words to say they have?
“She’s a daughter” is the answer but, “She is a daughter,” is rejected ... they mean exactly the same thing.