d. Before he went to sleep, he completed his homework.
Explanation:
A dependent clause is a group of related words that have at least a subject and a verb, but that do not express a complete thought or idea, and therefore, it can not stand alone as a full sentence. Sentence D is the sentence that has a dependent clause, which is <u><em>Before he went to sleep</em></u>. This latter has a subject (<em>he</em>) and a verb (<em>went</em>) and it does not express a complete idea on its own.
