Final answer:
A solid inner core and a liquid outer core spinning at differential speeds result in the creation of Earth's magnetic field due to circulating electric currents generated by the movement of liquid metal.
Step-by-step explanation:
What can exist as a result of a solid inner core and a liquid outer core spinning at differential speeds? The correct answer is A. Earth's magnetic field. This magnetic field is generated by the movement of material in Earth's liquid metallic core. As the liquid metal circulates, it sets up a circulating electric current, which results in a magnetic field. This magnetic field is crucial for many reasons, including protecting the Earth from solar winds that could strip away the ozone layer that protects us from harmful ultraviolet radiation.