Because of the difference in materials and the compression of gravity at depth, the inner core is the highest density. The lowest is the crust which is basically floating on the mantle. While no one has drilled that far, the basic assumption from magnetism and pieces of other planets delivered as meteors is that the inner material is iron, nickel and other heavy magnetic elements along with heavy radioactive elements.
The final answer to your question is C. core.
I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.
