Final answer:
The most common category of notification appliances is Audible, encompassing sirens, horns, and speakers designed to alert individuals in case of an emergency. D. Audible
Step-by-step explanation:
The most common category of notification appliances is Audible. Notification appliances are devices that alert individuals to emergencies, such as fire alarms or carbon monoxide detectors.
Audible appliances, like sirens, horns, and speakers, are most prevalent because they can quickly attract attention and are effective in a wide range of environments. Visual notification appliances, such as strobe lights, are also common but are typically used in conjunction with audible devices to ensure the message reaches everyone, including those who may be hearing impaired.