Introduction:
The Norton theorem is a technique used in electrical engineering to simplify complex circuits by reducing them to an equivalent circuit. It is named after Edward Lawry Norton, who developed this theorem in 1926. The Norton theorem is useful for analyzing and solving circuit problems by representing a complex circuit as a simpler one.

The Norton Theorem:
The Norton theorem states that any linear circuit containing independent sources and resistors can be replaced by an equivalent circuit consisting of a current source in parallel with an impedance. The Norton equivalent circuit is useful for simplifying complex circuits and analyzing their behavior.

Equivalent Circuit:
The Norton equivalent circuit consists of a current source in parallel with an impedance. The current source represents the current flowing through the original circuit, while the impedance represents the total resistance seen by the current source.

Option Analysis:
Let's analyze the given options to determine which one correctly describes the Norton equivalent circuit.

a) A current source only:
This option is incorrect because the Norton equivalent circuit consists of a current source in parallel with an impedance. It is not a current source only.

b) A current source with an impedance in parallel:
This option is correct. The Norton equivalent circuit consists of a current source in parallel with an impedance. This combination accurately represents the behavior of the original circuit.

c) A current source with an impedance in series:
This option is incorrect. The Norton equivalent circuit does not consist of an impedance in series with the current source. It is in parallel.

d) A voltage source only:
This option is incorrect. The Norton equivalent circuit does not consist of a voltage source only. It is a current source in parallel with an impedance.

Conclusion:
The Norton theorem reduces a circuit to a current source with an impedance in parallel. This equivalent circuit accurately represents the behavior of the original circuit and simplifies its analysis. The Norton theorem is a powerful tool in electrical engineering and is widely used to solve complex circuit problems.