Final answer:
A flame sensor detects flames and utilizes flame rectification to ensure safety by allowing gas flow only when a flame is present.
Step-by-step explanation:
The question "Flame is detected by what? Does it use flame rectification" is asking us to identify the device that detects flames and whether this process utilises flame rectification. The correct answer is B. Flame sensor, yes. A flame sensor is a device that's designed to detect the presence of a flame, ensuring that gas flows only when a flame is present to ignite it.
This safety mechanism prevents gas from being released into an environment where there is no flame to burn it, which could lead to dangerous situations such as gas leaks or explosions. Flame rectification is a method used in modern systems, where the sensor detects the presence of a flame by passing a current through the flame and checking for electrical conductivity, as a flame can rectify the current, meaning it allows current to flow in one direction.