"Are you from China?"
Translation:¿Eres de China?
It's not necessary and generally not used. It is not wrong to use it, though. Duo certainly uses it liberally!
If it's not in the database yet, I'd use the Report button next time. You don't want to fail a test out just because tú isn't set up for this sentence yet.
Why is it that estar is used to ask a question about someone's nationality, but ser is used to make a statement about someone's nationality? For example, I would use ser to say "you are American": Tú es americana. But if someone were to ask me "are you American," they would use estar: ¿Tú eres americana?
Here are the forms of estar: (yo) estoy, (tú) estás, (él, ella, usted) está, (nosotr@s) estamos, (ell@s, ustedes) están.
And the forms of ser: (yo) soy, (tú) eres, (él, ella, usted) es, (nosotr@s) somos, (ell@s, ustedes) son.
You've got them confused.
Ser is always used to ask about someone's nationality and to make a statement about someone's nationality.
You are American. = (Tú) eres americana (estadounidense).
Are you American? = ¿(Tú) eres americana (estadounidense)?
You might find studyspanish.com (Grammar Unit Two) helpful for ser/estar.