A smoke detector helps prevent a fire by detecting smoke and alerting the guests and staff to the danger, enabling quick evacuation and response.
- Fire safety sign is important for awareness, but it does not directly prevent a fire.
- Water sprinkler is useful for controlling or extinguishing fires, but the smoke detector is the primary preventive device in a guest room.
- Fire exit chart shows evacuation routes but does not prevent a fire.
Thus, the correct answer is Smoke detector.