In which situation is it NOT necessary to render a hand salute?

Rendering a hand salute is a sign of respect and recognition of military rank, and it is generally required in scenarios where military customs dictate it as a form of proper etiquette. However, when meeting an officer in a public transportation vehicle, such as a bus or train, it is considered inappropriate or impractical to perform a salute due to the nature of the environment and the constraints it imposes, such as limited space and the potential for safety hazards.

In contrast, the other situations mentioned in the choices—official ceremonies, being on a military base, and being in the presence of superior officers—are all circumstances where saluting is expected as a matter of protocol. Official ceremonies have established customs that necessitate saluting as part of the proceedings. On a military base, personnel are expected to adhere to uniform military traditions, including saluting when engaging with superiors. Similarly, when in the presence of superior officers regardless of location, military personnel honor the rank and position of those superiors with a salute. Thus, the unique nature of public transportation makes it the context in which saluting is unnecessary.