Answer: C. Cover letter
A job seeker must send a resumé and a cover letter to a potential employer to provide the necessary information. A cover letter must be strong to make a great impression where it allows more details from a candidate why they would be a great fit for the position, the need for a career change and gaps in their employment history.
Moreover, the educational certificate and work experience certificate are supporting documents that allows employers to confirm your claims and these are usually send if the potential employer would ask for it.
Lastly, the follow-up letter will only be send if they were no feedback in your application but nowadays they are not that popular or practiced.