Which of the following items is a common part of a fire sprinkler system?

A main drain is a critical component of a fire sprinkler system as it is essential for the maintenance and testing of the system. The main drain allows for the removal of water from the system, which is necessary during various procedures, such as inspections or repairs. It also plays a role in ensuring proper water supply and pressure within the system by allowing technicians to verify the system's functionality and to flush out sediments or debris that may have accumulated over time.

Other options provided, while important in different contexts, do not serve the same role within a fire sprinkler system. Security cameras are used for surveillance and safety purposes but don’t contribute directly to fire suppression. Emergency exits are vital for building safety, allowing occupants to evacuate safely, but they are not components of the fire sprinkler system itself. Electrical panels are necessary for the building's electrical infrastructure but do not pertain to the operation or maintenance of fire suppression systems. Hence, the main drain is the only choice that directly relates to the function and maintenance of a fire sprinkler system.