During the rendering of honors, what event cues the side boys to salute in unison?

The proper event that cues the side boys to salute in unison is when the boatswain's mate begins to pipe. The piping signal serves as a formal announcement that commands attention and indicates the time for the side boys to render honors. This traditional naval practice ensures that all personnel are aware of the dignitary's presence and highlights the importance of the event.

The other options, while important in the context of naval ceremonies, are not the specific cues for the side boys. The arrival of the captain on board is significant, but it is the boatswain's mate's piping that directly commands the salute. Similarly, though the national anthem is a crucial part of many ceremonies, it does not serve as the cue for the side boys. The raising of the colors is a formal event, but it pertains more to the salute to the flag rather than a specific prompt for side boys to salute.