The part of the sentence that is an adverbial dependent clause is option(a) when they arrived at the beach.
What is dependent clause?
An adverbial clause is a dependent clause that alters the primary verb in the independent sentence. Adverbial clauses always have a subordinating conjunction before them, and they need an independent clause to work correctly. For example, even if I take the train, I might not get at my appointment on time.
The three categories of dependent clauses are noun, adjective, and adverbial clauses. Reason-related adverbial clauses provide justification for the action conducted in the main clause of the sentence. Subordinating conjunctions such as because, unless, and since are frequently used in these sentences.