Answer: Sensor.
Answer analysis: This question mainly examines the basic concepts and terms in electrical systems. In electrical systems, each part and subsystem has its specific functions and roles. When we talk about "emitting a response signal to a specific form of energy", we are actually describing a system that can sense and respond to changes in a certain form of energy.
In electrical systems, such a system is usually a "sensor". A sensor is a device or element that can detect physical quantities and convert them into measurable signals.
These physical quantities can be temperature, pressure, light intensity, sound, magnetic field, electric field, etc., and the converted signal is usually an electrical signal, such as voltage, current or frequency.
Therefore, the sensor can emit a response signal to a specific form of energy.
In summary, the correct answer is "sensor".