Answer:
A. The clause is "that are not registered"
It is essential.
It does not require a comma.
Explanation:
Relative clauses are clauses that actually contain a subject and a verb. They are known to begin with a relative pronoun (e.g who, whose, whom, that or which) or a relative adverb (e.g when, why or where).
Option A is correct because "that are not registered" is actually a relative which uses "that" (a relative pronoun). Also, it is essential because it answers the question "what kind of cars are towed?". It does not require comma.
