If there are more than 20 sprinklers on a pre-action system, what must be installed?

In a pre-action sprinkler system where there are more than 20 sprinklers, it is crucial to install automatically supervised detection devices. This requirement is in place to ensure that adequate monitoring and control mechanisms are in place for the system to function effectively and to mitigate risks. Automatically supervised detection devices are designed to provide a higher level of safety, as they can continuously monitor for any changes in the system's status, such as the detection of heat or smoke, which would prompt the system to activate.

The importance of having such devices becomes particularly evident in systems with multiple sprinklers, as the increased number could lead to a more complex activation scenario. Keeping the system monitored helps in maintaining its integrity and ensures that it operates correctly during an emergency, providing the necessary response to a fire incident.

The other options do not directly address the need for monitoring and controlling a situation where a significant number of sprinklers are present. Thus, employing automatically supervised detection devices becomes essential in this context, ensuring both effectiveness and safety in the pre-action system.