The answer is B, adonde. If you use that choice, your sentence will mean: "To where are you going for the summer vacations? If you use De donde, your sentence will mean: From where are you going for the summer vacations?
Where is Javier from? Where does he live? When doesn't Javier swim? Where does Javier go on Mondays? What does Javier study? Where does Javier go after his classes?where do Javier and his friends go on Saturdays? What does Javier see in the museum?
