Norton’s theorem states that a linear bilateral network can be replaced with an equivalent current source in parallel with a resistor. The equivalent current source is equal to the short-circuit current at the terminals of the network, and the equivalent resistor is equal to the open-circuit voltage divided by the short-circuit current.
Option A is correct. Norton’s theorem replaces a complicated circuit facing a load by an ideal current source and parallel resistor. The ideal current source is equal to the short-circuit current at the terminals of the network, and the parallel resistor is equal to the open-circuit voltage divided by the short-circuit current.
Option B is incorrect. Norton’s theorem does not replace a complicated circuit facing a load by an ideal voltage source and parallel resistor.
Option C is incorrect. Norton’s theorem does not replace a complicated circuit facing a load by an ideal voltage source and series resistor.
Option D is incorrect. Norton’s theorem does not replace a complicated circuit facing a load by an ideal current source and series resistor.
Option E is incorrect. Norton’s theorem is a valid theorem that can be used to simplify the analysis of complicated circuits.