Answer:
A. Car
Step-by-step explanation:
Adjective clauses' function is to describe, modify or give more information about nouns. We can identify this type of clauses because they usually begin with a relative pronoun (that, who, whom, whose or which) or a relative adverb (when, where or why), and they are placed to the noun they describe. This is why the adjective clause “that is used for carpool” should be next to the noun “car” (the subject of the sentence), describing it.