Which of the following systems involves a condition of heat and smoke before activation?

The pre-action fire system is specifically designed to require a condition of heat and smoke before activation, distinguishing it from other fire protection systems. This system typically consists of a combination of a traditional sprinkler system and additional fire alarm components. Its operation requires that a fire alarm be activated—detecting heat or smoke—before the valves controlling water supply to the sprinkler heads are opened. This dual-trigger mechanism helps to minimize any potential water damage that could occur from accidental activation, making it particularly useful in environments where protecting sensitive equipment or materials is crucial.

This is unlike standard sprinkler systems, which activate automatically when the temperature reaches a certain threshold and do not rely on an alarm. Fire hose reels are manually operated and do not involve automatic activation based on heat or smoke. Water mist systems, while effective in various environments, typically activate based on either heat detection or manual initiation as well, and may not specifically require the combination of heat and smoke for operation in the same manner as pre-action systems. Thus, the characteristic of requiring both heat and smoke detection before releasing water makes the pre-action fire system the correct choice in this context.