Which of the following describes pre-action systems?

Pre-action systems are designed to require activation of a detection system before water flows into the pipes, which is why the correct choice highlights this aspect. In a pre-action system, two events must occur before water is discharged: a fire must be detected by the alarm system, and then the valve must open to allow water into the pipes. This provides an additional layer of safety, as the water is not in the pipes until there is confirmed evidence of fire, preventing potential damage from accidental discharge.

The other options do not accurately reflect the function of pre-action systems. For example, these systems do not activate instantly upon fire detection; there is a necessary step of valve activation before water is admitted. Additionally, pre-action systems are not limited to dry pipe technology; they can utilize wet pipe technology as well. Lastly, while they are designed to localize areas instead of spraying water over a large area automatically, they only discharge water in the immediate vicinity of the activated sprinklers upon fire detection, rather than employing a widespread automatic spray.