Which of these devices can be used to supply auxiliary power to alarm systems?

The correct answer is the battery charger. A battery charger is specifically designed to recharge batteries that provide backup power to alarm systems. In the event of a power failure, many alarm systems rely on battery power to continue operating. The battery charger ensures that the batteries remain charged and ready to supply this auxiliary power when needed.

While the control panel, circuit board, and keypad are essential components of alarm systems, they do not serve the function of supplying auxiliary power. The control panel manages the overall alarm system and processes signals, the circuit board houses the electronic components and connections needed for operation, and the keypad is used for user input, such as entering codes for arming or disarming the system. However, none of these components directly provide backup power; that role is dedicated to the battery charger.