Answer:
C is the correct answer.
Step-by-step explanation:
Hello, you want to ask two sisters so it cannot be A or D, as this is not plural.
B is not plural neither, just a formal way to say 'you'.
C is the correct answer.
'Vous' refers to the two sisters.
Comment vous appelez-vous?
Hope this helps.
Do not hesitate if you have any questions.