the presence of a fluid mantle the presence of Van Allen belts the presence of gravitational force the presence of water in three states
I gave my students this multiple choice question: Earth's core consists primarily of iron. Which observation about another planet would provide some evidence that the planet has an iron core?

Only one answer is correct among these 4 choices:
- the presence of a fluid mantle
- the presence of Van Allen belts
- the presence of gravitational force
- the presence of water in three states

Could you tell me which one is correct?

the presence of Van Allen belts