The correct answer is "D".
A cover letter must contain a brief introduction about yourself and then contain the reasons why you believe you are the person suited for the job. Contents like your previous experience in similar positions, the reason why you are applying and your qualifications all add to this objective.
On the other hand, employment history is information which should be exclusive content of your resume, as it is part of a more amplified set of information that the recruiter analyzes once he or she receives an application.