The correct answer is dynamo theory. The theory that explains the source of Earth's magnetic field is the dynamo theory. This theory proposes the mechanism of a celestial body such as Earth to generate magnetic field.
The Dynamo theory is a theory which talks about how a celestial body like earth generates a magnetic field. According to this theory the source of earth's magnetic field is a dynamo.
The concept that the magnetism of the earth is contributed by a dynamo was given by Joseph Larmor.
However, the presently accepted theory was perfected by W.M. Elsasser who said that the fluid present in the outer core of the earth is responsible for generating electric currents which in turn creates a magnetic field.
