Question

what is the evidence that the earth behaves as a magnet?​

Answer 1

The crust of the Earth has some permanent magnetization, and the Earth's core generates its own magnetic field, sustaining the main part of the field we measure at the surface. So we could say that the Earth is, therefore, a "magnet."

Answer 2

The Earth behaves like a magnet because the Earth is a magnet. It is not a permanent magnet, but an electromagnet. ... Deep in the Earth, molten metal (mostly iron) flows due to heat which causes convection. (The heat appears to be generated by iron condensing at the solid core.)