Which sensors all have short sensing ranges and are listed as detecting presence of objects and different materials?

• A. Hall effect, inductive, capacitive
• B. Proximity, Photoelectric, Ultrasonic
• C. Magnetic, Optical, Thermal
• D. Capacitive, Infrared, Ultrasonic

Answer: (A)

The main idea is that some sensors are built to work only at very close distances and are used to detect whether something is present and, in some cases, what kind of material is nearby. Hall effect, inductive, and capacitive sensors all sit in this near-field category. A Hall effect sensor detects magnetic fields, so when a magnet or a magnetized object is near, it produces a change that the sensor reads as presence—this happens at a short range. An inductive sensor generates a small magnetic field and detects changes caused by a metal object entering that field; because the interaction relies on the near-field magnetic coupling, the effective sensing distance is limited to a few millimeters up to a couple of centimeters. A capacitive sensor measures changes in capacitance between two electrodes; nearby objects or materials with different dielectric constants alter the electric field and capacitance, signaling presence within a small range. Together, they cover detecting metal, magnetic, and dielectric materials at close distances. Other options mix sensors that can operate over longer ranges or rely on different sensing principles, so they don’t all share the same short-range, presence-with-material-detection pattern.