The correct answer is B) The high limit control secures power to the entire oil firing system. When the desired steam pressure is obtained in an automatically fired auxiliary boiler, the high limit control activates to secure power to the entire oil firing system. This prevents the boiler from generating more steam than the desired pressure, which could be dangerous. The other options are incorrect because they do not accurately describe the actions that take place in the control circuit under these conditions. A temperature sensing device does not open the circuit breaker, the low limit control does not break the ignition circuit, and the stack relay does not secure power to the ignition transformer.