The correct answer is D: Field Effect Transistor. FET stands for Field Effect Transistor, a type of transistor that controls current flow using an electric field rather than current injection (as in bipolar junction transistors). FETs include MOSFETs (Metal-Oxide-Semiconductor FETs) and JFETs (Junction FETs), and they have three terminals: gate, drain, and source. The gate terminal controls current flow between drain and source by creating an electric field that modulates the conductivity of a channel. FETs are widely used in modern electronics because they have high input impedance, low noise, and excellent high-frequency performance. For amateur radio operators, FETs are commonly found in RF amplifiers, mixers, oscillators, and switching circuits in transceivers and other equipment.