Which of the following is a formal expression of respect when addressing an officer?

A formal expression of respect when addressing an officer is "Sir." This term is a traditional and standardized way of acknowledging the rank and position of military personnel, particularly in the Navy and other branches of the armed forces. Using "Sir" shows respect and maintains a professional demeanor that is essential in military culture. It reinforces the chain of command and indicates recognition of the authority held by the officer.

Terms such as "Hey," "Yo," or "Dude" are casual and informal, which are not appropriate in a military setting when addressing someone of higher rank. These alternatives lack the respectfulness and formality required in military communication. Thus, using "Sir" not only conveys respect but also adheres to the customs and courtesies expected within the military environment.