Choose the most suitable one word for the following phrase. Informal business communication with a personal signature

Memorandum is the correct word.
A memorandum is a note that records something for future use, usually commercial communication, and includes the signatures of the people involved. For example, "he told them of his decision in a memorandum". As a result, Option 4 is right. Among the other alternatives, agendumrefers to a plan or list of things to be done, corrigendum refers to something to be rectified, generally an error in a printed book, and plagiarism refers to the practise of taking someone else's work or ideas and passing them off as one's own. These alternatives do not express this notion and are hence incorrect.