The answer is C. submit only the cover letter and résumé.
A cover letter is a document that is sent together with the CV (curriculum vitae), and the purpose of this letter is to make a case of <em>why you should be hired</em>. It distinguishes you from other candidates, by allowing you to express how interested you are in the position; also, it's a <em>good opportunity to to highlight your relevant qualifications.</em>