Which safety device opens if there is a crack in a heat exchanger?

The rollout switch is the correct choice because it is specifically designed to provide safety in the event of abnormal conditions within the heating system, such as a crack in the heat exchanger. When the heat exchanger cracks, it can lead to combustion gases escaping, which can be dangerous. The rollout switch is positioned in a way that it can detect these gases or the excessive temperature that might result from an unsafe condition. If it senses the presence of these gases or high temperatures, it will open to shut down the system, preventing the potential hazards associated with a compromised heat exchanger.

Other safety devices mentioned, like the limit switch, are important but serve different functions. Limit switches typically are used to prevent overheating by monitoring the temperature of the heat exchanger and shutting off the burner if the temperature exceeds a set limit. The safety switch might refer to other generalized protection devices found in systems, while the pressure switch is primarily utilized to monitor the pressure within the system and does not respond specifically to cracks in the heat exchanger. Thus, the rollout switch is the critical safety device that directly responds to the specific scenario presented in the question.