Final answer:
Mars' magnetic field is weak likely due to its core no longer being molten and potentially containing less iron, preventing the generation of significant electric currents needed for a strong magnetic field. Option D is correct.
Step-by-step explanation:
The question asks why Mars' magnetic field is so weak. The most likely reasons are that Mars' core may no longer be molten which inhibits the generation of a significant magnetic field, and its core contains less iron compared to Earth's core.
On Earth, the magnetic field is generated by the movement of liquid metal in its core, creating circulating electric currents. Since Mars' magnetic field is considerably weaker, it suggests that its core might not be in a liquid state anymore, which means electric currents aren't generated to produce a strong magnetic field.
Additionally, if Mars has less iron in its core, this could also contribute to a weaker magnetic field since metals like iron are key in the magnetic properties of planetary cores.