A photoelectric sensor uses the principle of ... to activate it.

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Light changes the electrical conductivity of a photosensitive material, and that change is what activates the sensor. In a photoconductive setup, illumination creates more charge carriers in the material, lowering its resistance. This allows current to flow and the sensor’s output to switch or trigger. The photovoltaic effect, by contrast, would generate a voltage from light rather than primarily changing resistance to enable current flow; photoemission involves ejecting electrons from a surface, which isn’t how a typical photoelectric sensor is activated; and a photoresist is a light-sensitive coating used in patterning, not a sensing mechanism. So the activation relies on photoconduction—the light-induced increase in conductivity.