Which description correctly represents Thevenin's theorem for a linear circuit as seen from two terminals?

• A. A current source in parallel with a resistor
• B. An inductor in series with a capacitor
• C. A transformer attached to the terminals
• D. A voltage source in series with a resistor

Answer: (D)

The main idea being tested is that any linear circuit seen from two terminals can be replaced by a Thevenin equivalent: a voltage source in series with a resistor. This form fully captures how the network behaves for any load connected across those terminals. The voltage source value (Vth) equals the open-circuit voltage across the terminals, and the series resistance (Rth) is what you’d measure looking into the terminals with all independent sources turned off. With a load, the current becomes I = Vth / (Rth + Rload) and the terminal voltage follows Vload = Vth · (Rload / (Rth + Rload)). So the description of a voltage source in series with a resistor matches Thevenin’s representation exactly. The other descriptions describe different concepts (for example, a current source in parallel with a resistor is the Norton form; an LC pair or a transformer involves energy storage or magnetic coupling, not a fixed Thevenin equivalent).