Answer:
¿De qué nacionalidad eres?
Step-by-step explanation:
The correct form to ask the exchange student in your class about her nationality is to say '¿De qué nacionalidad eres?' because 'eres' refers only to one person (singular).
The other options ('¿De dónde vienen?', '¿De qué nacionalidad son?', '¿De qué nacionalidad vienen?') all refer to more than one person. You can realize it with the words in plural: 'vienen' and 'son'.