I think it is C, because if I'm right present principal is something that is happening presently? If so, C would be the answer because they "are doing" the exercises right now in context.
Nothing is underlined in your sentence, but the sentence sounds like it is set in the present.
B. present
A thesis statement focuses your ideas into one or two sentences. It should present the topic of your paper and also make a comment about your position in relation to the topic. Your thesis statement should tell your reader what the paper is about and also help guide your writing and keep your argument focused.
