Final answer:
The preposition 'dans' best completes the sentence by indicating the location of the car in the garage.
Step-by-step explanation:
The preposition that best completes the sentence is dans. It means 'in' in English. So the sentence would read 'The car is in the garage.' This preposition indicates the location of the car.
Example: Je suis dans la maison. (I am in the house.)
When talking about the location of something, the preposition 'dans' is commonly used in French.
Learn more about Prepositions in French