The question tests the understanding of sentence structure. The correct answer is 'C. To express a complete thought on its own' because a main clause can stand alone as a sentence and expresses a complete thought with a subject and a verb. 'A. To provide additional information about the subject' is the role of a relative clause. 'B. To act as a subordinate to another clause' describes a subordinate clause. 'D. To introduce a list of items' is typically the role of a colon or a phrase like 'such as'.