A wet sprinkler system is characterized by being constantly filled with water. This design allows for immediate response to a fire, as the water is readily available at the sprinkler heads when the system is activated. When heat from a fire triggers the sprinkler heads, the water is discharged directly onto the fire, helping to suppress it quickly and effectively.
The choice of systems that remain empty until activated refers to dry sprinkler systems, which have pipes that are filled with air or nitrogen until the system is activated, at which point the air pressure drops and water flows into the system. Meanwhile, options describing systems that remain pressurized with air pertain to dry sprinkler systems as well, which are specifically designed to prevent freezing in unheated areas. Lastly, options involving delayed response mechanisms could relate to other specialized systems that are designed not to activate immediately, but this is not a characteristic of wet sprinkler systems.