The correct answer is A. The clause is "that are not registered." It is essential. It does not require a comma.
Step-by-step explanation:
In grammar, relative clauses are used to define a noun. These can be identified because they begin with words such as that, when, where, etc. In the sentence presented the relative clause is "that are not registered", which defines "the cars".
Moreover, relative clauses are essential if they provide key information and cannot be omitted. This occurs in the clause "that are not registered" because without this it is not possible to know the cars that will be towed. Also, according to grammar rules, essential clauses do not require commas.