The choice that best describes the purpose of the underlined adverbial clause in the sentence is B. The adverbial clause indicates reason.
Explanation:
Adverb clauses, also called adverbial clauses, are dependent clauses that function as adverbs. As they are dependent clauses, they must have a a subordinating conjunction to connect them to the rest of the sentence. In the sentence above, the subordinating conjunction introducing the adverbial clause is <em>because. </em>It gives information as regards the reason why the speaker decided to wait in the long line.
