Final answer:
The correct preposition to fill in the blank is 'au'.
Step-by-step explanation:
The correct preposition to fill in the blank in the sentence is au. The sentence should be: Cet été pour les vacances, je vais au Sénégal. The preposition 'au' is used to indicate movement towards a specific country. In this case, it indicates that the person is going to Senegal for summer vacation.
Learn more about Prepositions in French