Each type of sensor uses a different electrical principle to sense presence. Which option correctly completes the statement?

• A. Mechanical
• B. Optical
• C. Thermal
• D. Electrical

Answer: (D)

The idea being tested is that sensing presence is described by the physical phenomenon the sensor relies on, and when we talk about an electrical principle, we’re pointing to how the sensor’s detection mechanism operates in electrical terms. Electrical sensing covers changes in electrical properties—like capacitance, resistance, or inductance—caused by a nearby object or presence. Those changes are what get turned into an electrical signal the system can read. The other terms describe different kinds of physical effects (mechanical contact, light, heat) rather than a direct electrical principle used to sense presence. So the statement is completed with the electrical option, indicating the sensing relies on electrical phenomena.