This question is still waiting for an answer, please, how to ask "Are you (female) an/the only CHILD (offspring, issue)?" Not "Are you the only daughter (possibly with male siblings)?" Thank you.
(PS I'd use 'the' when conversing in the context of '-of your parents'' but 'an' in the context of classification in the world at large.)
"Sei figlio unico?" or "Sei figlia unica?" indicate basically Are you the only child? (If you are "Figlio unico" or "figlia unica" you cannot have silbling). If you want to ask "Are you the only female/male child?" you must specify and the sentence will be "Sei l'unica figlia femmina?/Sei l'unico figlio maschio?" Used normally when you know that your listener have al least a sibling of the other sex. "Sei UN figlio unico"!"/Sei UNA figlia unica"!"" indicate that the "figlio unico/figlia unica" is special and there are not another one like him/her. "Sei UN figlio unico"?"/Sei UNA figlia unica"?"" have the same means of "Sei figlio unico?/Sei figlia unica?" or "Are you a special son?" (the second meaning is unusual) "Sei l'unico figlio?/Sei l'unica figlia?" The parent/parents is/are implied. E.g. Sei l'unico figlio di Marco? Are you the only child of Mark? Sei il figlio unico?/Sei la figlia unica?" Means "Are you The Only (specific of famous) child? (unusual)
Sorry for my terrible English, I'm not native speaker (I am Italian). I hope to be understandable.
"Are you an only child?" is certainly best, but it also listed "Are you an only son?" as an a acceptable response. While a literal translation, "an only son" isn't something anyone would say in (American) English. It sounds like something a foreigner would say that is grammatically correct but doesn't really make much sense. The intended meaning of that phrase is presumable "Are you the only son?"
"Are you an only child?" should probably be the only option.
That would mean something different in English, since 'unique' has a different meaning. Unique means that something is different or unlike the others. So if someone were the unique child, maybe they would be tall when everyone else is short, or blonde instead of brunette, in a family of brunettes.
Hunter...It means 'you're an only child' with 'figlio' (son) standing for 'child', just as the plural 'figli' can mean both 'sons' and 'children/sons & daughters'. So 'you're 1 only son' makes absolutely no sense whatsoever and shouldn't be considered regardless of what DL might provide as an alternative.