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A
Stores energy
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B
Amplifies signals or acts as an electrically-controlled switch — a small input current or voltage controls a much larger output current
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C
Converts AC to DC
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D
Produces magnetism only
Why this is the answer
Transistor: semiconductor device that AMPLIFIES or SWITCHES electronic signals. Invented at Bell Labs in 1947 (Bardeen, Brattain, Shockley — Nobel Prize 1956). Foundation of all modern electronics. Two main types: (1) BJT (Bipolar Junction Transistor) — three terminals: Base, Collector, Emitter; small current at base controls larger current between collector and emitter; comes in NPN and PNP polarities; (2) FET (Field-Effect Transistor) — three terminals: Gate, Drain, Source; voltage at gate controls current between drain and source; very high input impedance (doesn't draw current at gate); MOSFETs are most common — used in nearly all digital electronics. As AMPLIFIER: small input signal (e.g., audio from microphone, signal from antenna) varies the larger current flowing through the transistor; output is amplified version of input. Used in audio amplifiers, radio receivers/transmitters, RF circuits. As SWITCH: small base current (BJT) or gate voltage (FET) turns the transistor fully ON or fully OFF; replaces mechanical switches in electronic circuits; very fast (billions of switches per second); essential for digital electronics. Integrated circuits (ICs) pack millions to billions of transistors on a single silicon chip: microprocessor (modern CPU) has tens of billions of transistors; memory chips have similar counts. Moore's Law (Gordon Moore, 1965): transistor density on chips doubles approximately every 2 years — held remarkably true for decades. Modern transistor gate sizes: 3-5 nanometers (about 10-20 atoms wide). Transistor configurations: COMMON EMITTER (amplifies both current and voltage), COMMON BASE (amplifies voltage), COMMON COLLECTOR / EMITTER FOLLOWER (amplifies current). Identification: schematic symbol shows arrow indicating emitter direction (NPN arrow out, PNP arrow in). Transistor ratings: maximum voltage, maximum current, maximum power dissipation, gain (hFE for BJT, transconductance for FET). Common BJTs: 2N2222, 2N3904 (general purpose), 2N3055 (power). Common MOSFETs: 2N7000, IRF series (power switching).
Source: ASVAB EI, Transistors