The correct answer to which clause from the sentence above is a dependent clause is option C) while I was reading last night.
A dependent clause is a group of words with a subject and predicate but that do not express a complete thought, so it cannot form or function syntactically as an independent sentence. It can be joined to another sentence to express a complete thought and function as an adverb, adjective or noun clause in a complex sentene.
In this case, while I was reading last night is the dependent clause because it cannot function as an independent sentence. We need more information about what happened while the speaker was reading last night. Thus, this clause is a dependent clause that functions as an adverbial phrase when combined with a main clause in a complex sentence.
