A. Reference. B. Assertive closing. C. Greeting. D. Body of letter
I'd say A. Reference
It is recommended to have an assertive closing when writing a cover letter and do not become too aggressive. Cover letter openings should have formal greetings/salutations such as Dear Ms. or Dear Mr. What follows next after the salutation is the body of the letter. Do not include references in your cover letters unless the prospective employer asks you to.