The best answer is option 4.
On Earth, the record of the reversal of the magnetic field is preserved in magnetic rocks which lie along the ocean floor. The magnetism preserved in these rocks point first in one direction then in another direction, giving the ocean floor a stripped appearance (from a magnetic point of view)
Earth's magnetic field is powered by the core of the planet. At its center, is a solid inner core surrounded by a fluid outer core that is hotter at the bottom. Hot iron rises within the outer core then cools and sinks. The convention currents combined with the rotation of the earth are thought to generate a geodynamo that powers the magnetic field.
