Answer:
a. Letter
Step-by-step explanation:
A letter is written communication from one party to another. Letters are written on a designated paper, put in an envelope and send either by post or hand-delivered.
A letter can be formal or informal. Daniel should write a formal letter to his boss informing him of his resignation. Should Daniel write a letter, It will an official document in the company. The company will be file the document appropriately and will stay intact for many years.
Daniel can also keep a copy of the letter for himself. A letter has an advantage over the other electronic options. Electronic records can be deleted permanently or get lost should the systems collapse.