Final answer:
Infrared and Ultrasonic sensors can detect an object entering their field; an Infrared sensor detects infrared emissions, while an Ultrasonic sensor uses sound reflection. Temperature and Pressure sensors are not designed to detect object presence without direct effect.
Step-by-step explanation:
The sensors that can detect an object entering their field are the Infrared sensor and the Ultrasonic sensor. An Infrared sensor measures the infrared radiation emitting from an object, which varies with temperature. This type of sensor is commonly found in infrared cameras and devices like ear thermometers. The Ultrasonic sensor emits sound waves that reflect off objects; the sensor then detects the time it takes for the waves to return, as used in automatic focus cameras and sonic range finders. Meanwhile, a Temperature sensor like a thermistor measures temperature changes by the variation in resistance, but it does not specifically detect objects entering its field. A Pressure sensor responds to pressure changes on its surface from direct contact with an object, which may not necessarily have entered its field sensor.