Answer:
C. A follow-up letter
Step-by-step explanation:
A follow-up letter offer presents a chance for the interviewee to say thank for the interview opportunity. It shows gratitude and good manners on the part of the interviewee. A follow-up letter is an opportunity to give additional information that one might have omitted in the interview.
After an interview, a follow-up letter reminds the interviewer about the interviewee. Even if that interviewee does not get that particular position, the interviewer will have them in mind in case of other opportunities.