What occurs at the last note of Retreat on military installations?

The correct understanding of what occurs at the last note of Retreat on military installations is that a gun is fired, followed by the national anthem or "To The Color," and the flag is lowered. This practice serves several purposes: it honors the national flag and the country, signifies the end of the duty day, and reinforces the tradition and respect associated with military protocol.

The ceremonial act begins with the firing of a gun, which is a signal that follows the musical notes. The subsequent playing of the national anthem, or "To The Color," is a moment that calls for all personnel on the installation to show respect either by saluting or by standing at attention as appropriate. Lowering the flag at this time symbolizes the end of the day and is a significant aspect of military customs that marks a moment of reflection and respect for those who have served.

The other options do not accurately describe the correct sequence of events during Retreat and miss the key aspects of flag protocol and military traditions associated with this ceremony.