Which one of the following types of writing is considered formal?

A. An instant message to a co-worker about after-hours plans

B. An invitation to an anniversary party

C. A personal email to a friend

D. A cover letter with a job application

D. You are having to communicate revolving around why you as a person should be in that position.

D. i think its D because if youre going to apply for a job, A cover letter accompanies a CV. It's an opportunity to highlight what's in your CV and to provide real examples to support your ability to do the job. So it is the most formal of all answers.