What changes its resistance based on the amount of light it senses?

The device that changes its resistance based on the amount of light it senses is the cad cell. A cad cell, or cadmium sulfide photoresistor, is commonly used in oil burner applications to detect the presence of a flame. Its resistance decreases as light levels increase, which allows it to generate an electrical signal that can indicate whether the burner is producing a flame or not.

In the context of oil burners, the cad cell provides critical feedback to the burner control system. If the flame goes out or is insufficient, the change in light detected by the cad cell leads to a rise in resistance, signaling the control system to shut down the burner to prevent the accumulation of unburnt fuel.

The other devices listed serve different purposes. A thermocouple is used to measure temperature; it generates a voltage in response to temperature differences but does not respond to light. A phototransistor is similar in function to a cad cell, as it is also sensitive to light, but it primarily operates as a switch or amplifier for electrical signals rather than changing resistance based on light levels. A pressure switch, on the other hand, detects changes in pressure and responds accordingly, unrelated to light detection.

Thus, the cad cell is specifically designed to alter its resistance