Which of the following is an example of an active fire protection system?

An active fire protection system is one that requires some form of activation, usually through a manual or automatic process, to provide protection during a fire event. Sprinkler systems fall into this category as they are designed to be activated when a fire is detected, typically through heat from the flames. When the temperature reaches a certain threshold, the sprinkler heads release water to suppress or extinguish the flames, actively working to mitigate the fire's impact.

In contrast, other options like fire alarms are also considered active systems since they detect smoke and fire, but they do not suppress the fire. Firestop barriers and smoke curtains, while important for fire safety, primarily serve as passive fire protection measures by delaying the spread of fire and smoke without direct activation in response to a fire incident. Thus, sprinkler systems clearly exemplify active fire protection as they engage directly with fire suppression when triggered.