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Flame ionization is the idea here: in a hydrocarbon flame, hot gases produce free ions and electrons, creating a conductive path. A flame rod acts as an electrode placed in the flame, and the flame itself conducts a small current between the rod and a sensing circuit. The control system checks for this ionization current to confirm that a flame is present; if the flame is out, the current drops to nearly zero and the gas supply can be shut off for safety.

This is why the flame rod is the best answer—it directly uses the ionized flame to carry current and signal presence. The other options don’t rely on ionization conduction: a thermocouple measures temperature and generates a voltage from heat, not flame ionization; a cad cell (cadmium sulfide cell) detects flame light by changing resistance, not electrical conduction through ionized gas; an ion sensor is a broader term and not the specific flame-sensing method described here.