What indicates that the weapon is safe when the selector lever is in the safe position?

The indicator that the weapon is safe when the selector lever is in the safe position is that the hammer will not fall when the trigger is pulled. This feature is a fundamental safety mechanism in firearms design, providing assurance that the weapon cannot fire inadvertently while the selector is set to safe.

When the selector lever is engaged in the safe position, it disconnects the trigger from the firing mechanism, preventing any action that would allow the hammer to strike the firing pin. This design is critical for maintaining the safety of the weapon, especially in environments where accidental discharge must be avoided.

While the other options touch on aspects of firearm handling and safety, they do not directly indicate the safety status of the mechanism in relation to the selector lever's position. For instance, a magazine being removable or the rifle being unloaded does not necessarily mean that the weapon is safe for handling, as it does not address the firing mechanism's status. Similarly, while the bolt remaining locked to the rear may provide an added safety measure, it does not guarantee that the weapon is incapable of firing if the trigger were to be pulled. Therefore, the primary indicator of safety in this case is the inability of the hammer to fall when the trigger is engaged in the safe setting.