Which statement is true about a 9V battery in a sentinel device?

The statement that a 9V battery must be used as the only power source in a sentinel device is correct because sentinel devices are specifically designed to operate with a designated power supply that meets their voltage and current requirements. Using a 9V battery ensures that the device functions properly and safely, as the device’s circuitry is engineered to work with that particular voltage level. Substituting the battery with other voltage types, such as 6V, or different battery types can lead to malfunction, damage the device, or compromise safety. This reliability is crucial in mining environments where proper functioning of safety equipment can be a matter of life and death.

In contrast to this option, the other choices present inaccuracies regarding compatibility and performance. For instance, using a different battery voltage could significantly affect the device's performance and lead to operational failures.