A cover letter is:

a$)$ A formalized framework consisting of a set of question$/$scales designed to generate primary raw data

b$)$ Always given to the participant during the first face to face encounter with the researcher

c$)$ A formalized statement of a testable relationship between two or more constructs or variables

d$)$ A response to a respondent sent to confirm receipt of their completed questionnaire

e$)$ A separate written communication to a prospective respondent designed to enhance that person's willingness to complete and return the survey in a timely matter