What command may any cadet give when a guest enters the classroom?

When a guest enters the classroom, the appropriate command to give is "Attention on deck!" This command is used to signal all cadets to stand in a position of respect and readiness when someone of authority or a guest arrives. It promotes an atmosphere of discipline and decorum, reflecting the values instilled in cadets regarding respectful behavior towards guests and superiors.

The other commands have different contexts and applications. "Stand at ease!" is used to relax from a position of attention but does not convey the same level of respect as the correct command. "Eyes front!" instructs cadets to focus their attention forward, but it does not specifically address the presence of a guest. "Fall out!" instructs cadets to disperse from formation, which would be inappropriate in a situation where a guest is being welcomed into the classroom. Thus, "Attention on deck!" is the most fitting response in this context.