Which of the following is an example of a passive fire protection system?

Passive fire protection systems are designed to contain fires and prevent their spread without the need for active intervention. This includes the use of building materials and construction methods that create barriers to fire and smoke. Fire-rated walls and doors are specifically engineered to withstand fire for a certain period, thereby protecting vital areas of a building and allowing occupants additional time to evacuate safely. These structures are essential in compartmentalizing a building, slowing down the fire spread, and maintaining the integrity of escape routes.

In contrast, the other options represent active fire protection measures. A sprinkler system and fire hydrant require a source of energy or human intervention to operate and actively suppress flames. Smoke detectors are part of an active alarm system that alerts occupants to the presence of smoke but does not prevent fire spread on their own. Thus, fire-rated walls or doors stand as the definitive example of passive fire protection, providing crucial barriers against fire and smoke.