The main function of a conjunction is to join or combine two clauses as a statement. In the given sentence, the clauses are identified as "He refused to do his homework" and "he thought he would not be able to do it". The latter is a subordinate/dependent clause, as it depends on the former clause, which is the main/independent clause for its full meaning. The word left behind is 'because', without which the clauses are grammatically complete, but they do not sound as a complete sentence. Hence, "because" is a conjunction that is used for cause-related sentences "he thought he would not be able" which is the cause for the action "refused to do homework" in the main clause. Hence, option C is correct. 'He' is a pronoun in the third person singular form, as pronouns modify the nouns which have been identified with or mentioned before. It does not combine the clauses and without it the main clause is grammatically incomplete. Hence option A is incorrect. 'Would' is a modal auxiliary as it determines the mood or tense of the sentence, it does not combine the clauses, hence it's not a conjunction and option B is incorrect. 'Do' is a verb that means 'to perform an action', it refers to an action performed, and does not combines the clauses, hence option D is incorrect.