The transistor providing a high signal at its output when the sensor is energized, or has detected the presence of an object.

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The concept being tested is sourcing vs sinking outputs in transistor-based sensor circuits. When the sensor detects something and the output goes high, the transistor is sourcing current from the positive supply to the load. A PNP transistor does exactly this: its emitter ties to the positive supply, and when the sensor energizes it forward-biases the base-emitter junction so current flows from emitter to collector to the output, pulling the output up toward the supply voltage. That’s why the output is high when the sensor is energized.

In contrast, activating an NPN transistor (or an N-channel device) would sink current to ground, pulling the output low rather than high. A P-channel device can also source current in high-side switching, but since the prompt refers to a transistor and the intended convention in many sensor outputs is PNP for a high (sourcing) output, the best match is PNP.