Which material in a cad cell allows it to respond to changes in light?

The cad cell, or cadmium sulfide photoresistor, is a crucial component in an oil burner control system. It functions by responding to the presence or absence of light, making it essential for the safe and efficient operation of the burner. The material that allows this light response is cadmium sulfide. This semiconductor compound changes its resistance based on the amount of light it detects; when light levels rise, the resistance decreases, allowing more current to flow through the circuit. This property is utilized in the cad cell to monitor if the flame is present during the operation of the burner, thereby ensuring safe combustion processes.

The other options do not have the same light-responsive properties as cadmium sulfide. Silicon, while also a semiconductor, does not respond to light in the same manner as cadmium sulfide; carbon does not have significant photoconductive properties; and chlorine is a gas with no relevance to light sensitivity in the context of a cad cell. Thus, cadmium sulfide stands out as the correct answer due to its specific behavior in relation to light changes.