Obsidian,(underlined: which was used by early man to make arrowheads), is a naturally occurring glass.
A.
essential adverb clause
B.
nonessential adjective clause
C.
nonessential adverb phrase
D.
essential adjective phrase

The correct answer is B: nonessential adjective clause.

Explanation:

Adjective clauses are dependent clauses that provide us more information about a noun or pronoun in a sentence, by acting as an adjective in that sentence. Adjective clauses can be essential and nonessential.

Essential adjective clauses contain some important details about the sentence, so they cannot be removed from the sentence, as the sentence wouldn't make sense without it.

On the other hand, nonessential adjective clauses contain nonessential information, so they can be removed from the sentence without changing its meaning. These types of clauses are separated by commas.

do you have more to the question?

one is a red flower, and one is the sky when the sun sets.