Explanation:
D.O. letter stands for Demi Official letter. It is used for official communication with a personal touch, generally between officers. It is less formal than a standard official letter but still relates to official matters. Questions on office correspondence are relevant for assistant posts because assistants may assist in drafting letters, reminders, notes, and reports. Candidates should know common official communication formats such as office memorandum, circular, notice, reminder, report, and demi official letter.