Answer: The correct answer is C. Tiene.
Explanation: By putting "tienes" you would be saying: How old are you?
and by putting "eres" you would be saying: How old you are?
Neither of those would make since because you are talking about a different person.
The answer is
C. Tiene
Because when you put it together in English it says this
“Your sister is so young. How old is she?”
So the translation to that is.
“Tú hermana es muy joven. ¿Cuantos años tiene?
