Read this sentence.

All cars that are not registered will be towed to the nearest impound lot.

Identify the relative clause, whether it is essential to the meaning of the sentence, and if it requires commas.

a.
The clause is “to the nearest impound lot.”

It is non-essential.

It does require a comma.

b.
The clause is “that are not registered.”

It is non-essential.

It does require a comma.

c.
The clause is “that are not registered.”

It is essential.

It does not require a comma.

d.
The clause is “to the nearest impound lot.”

It is essential.

It does not require a comma.