The correct answer is: A. Transformer
A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. It does this by using electromagnetic induction to create a magnetic field that induces a voltage in a second circuit. Transformers are used in a wide variety of applications, including power transmission, distribution, and electronics.
A transponder is a device that receives a signal and retransmits it, usually with a change in frequency. Transponders are used in a variety of applications, including radar, satellite communications, and air traffic control.
A transducer is a device that converts one form of energy into another. Transducers are used in a variety of applications, including microphones, speakers, and sensors.
A converter is a device that changes the form of electrical energy. Converters are used in a variety of applications, including power supplies, inverters, and rectifiers.
In the context of larger buildings that are supplied with a medium voltage electricity supply, the main item of equipment contained in a substation or mini-sub is a transformer. The transformer steps down the medium voltage electricity to a lower voltage that can be used by the building’s electrical system.