Which theorem states that any source can be reduced to a single equivalent voltage source in series with a resistor?

• A. Kirchhoff's Law
• B. Norton's Theorem
• C. Principle of Superposition
• D. Thevenin's Theorem

Answer: (D)

The idea being tested is Thevenin's theorem: any linear network of sources and impedances seen from two terminals can be replaced by a single voltage source in series with a resistor. This makes analysis easier because you only need two values—the Thevenin voltage (the open-circuit voltage across the terminals) and the Thevenin resistance (the equivalent resistance seen by the terminals with all independent sources deactivated). For networks with dependent sources, you determine the Thevenin resistance by using a test source to probe the circuit rather than simply turning sources off.

This is why the statement points to a voltage source in series with a resistor as the equivalent form. It’s a way to encapsulate the entire network’s behavior into a simple two-element model that yields the same terminal behavior for any load.

Other options don’t fit this specific form. Kirchhoff’s laws describe how currents sum at a node and how voltages sum around a loop, not how to condense a network into an equivalent single source and series resistor. The principle of superposition tells you how to analyze the circuit by adding responses from individual sources, but it doesn’t provide a single equivalent source-and-resistor representation for the whole network. Norton’s theorem gives an equivalent current source in parallel with a resistor, which is a related but different form; Thevenin’s theorem specifically describes the voltage-source-in-series-with-a-resistor representation.