Informal business communication with a personal signature

Answer: Option D

One word substitute is Memorandum.

Agendum: a schedule or list of items to be attended to.
Corrigendum: a thing to be corrected, typically an error in a printed book.
Plagiarism: the practice of taking someone else's work or ideas and passing them off as one's own.
Memorandum: a written message in business or diplomacy.