Right answer:
b. de las
Here we use the preposition de and the definite article las. This preposition is used to indicate possession and we use it here because the car belongs to the sisters. Recall that a preposition is a word (or group of words) which is used to show the way which other words are connected, that is, they are used to indicate a relationship between two words or phrases, between nouns, between nouns and verbs, and between different parts of a sentence. On the other hand, since hermanas is a feminine plural noun, we must use the feminine plural definite article, which is las. Finally:
El auto es de las hermanas