Answer:
An adverb clause of reason
Step-by-step explanation:
Dependent clauses are clauses that don't express a complete thought, because of which they can't stand alone. Unlike independent clauses, without the rest of the sentence, they don't mean anything. There are three types of dependent clauses: noun, adjective (relative), and adverb clauses.
Because he was very tired is an adverb clause. Adverb clauses are groups of words (clauses) that function as adverbs. They modify verbs, providing additional information about when, where, why, how, how much or under what condition the action expressed the verb takes place. The clause because he was very tired informs us about the reason, about why the action from the main clause is performed. That's why it's an adverb clause of reason. These clauses usually begin with conjunctions such as because, since, as, and that.