The correct answer is C. Many of the candidates.
The subject in a sentence is the noun or pronoun that is doing or being something. The verb expresses the action that the subject is doing or being. I can be a simple subject, a complete subject and a compund subject.
A simple subject is only a noun or pronoun. A complete subject includes all the modifiers. A compound subject includes more than one subject--for example, when there are two people performing an action.
In this sentence, the candidates are the ones who are speaking, so the simple subject is "candidates" and the complete subject is "Many of the candidates".