They restedwhen evening came.
The underlined group of words in the above sentence is a/an:

Let us explore the given options: 'Adjective clause' also known as a relative clause, is a type of dependent clause that works to describe a noun in a sentence. In the case of an adjective clause, all the words work together to modify the noun or pronoun.
'Noun clause' is a dependent clause that acts as a noun.
Noun clauses begin with words such as how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, and why. 'Adverb clause' is a group of words that function as an adverb in a sentence, modify or describe verbs, adverbs, and adjectives.
It adds information that elaborates on when, where, why, how, how much, or under what condition the action in the sentence takes place.
'Conjunction clause' is a group of words that function as a conjunction that holds words, phrases, and clauses (both dependent and independent) together. There are three different kinds of conjunctions - coordinating, subordinating, and correlative - each serving its own, distinct purpose, but all working to bring words together.
In the given sentence, the phrase ‘when evening’ came is modifying the verb ‘rested’.
So this clause is an adverb-clause. Hence, the correct option is option 3.