Answer: D. the immense pressure that it is under
Explanation:
The Inner Core is made up of iron and experiments have shown that when the pressure on a solid such as Iron increases, so does it's melting point. The Earth's inner core is under a lot of pressure from the Mantle as well as the outer core.
This pressure increases the melting point of the iron to a point where it is higher than the temperatures in it which causes it to remain in a frozen state.
