Which layer is a solid made mostly of nickel and iron? a) crust b) astenosphere c) inner core d) none of the above

C. The inner core is known to be made mostly of nickel and iron. However, since there is an outer core (which is the original one that's made out of nickel and iron), your answer might be D. But if you put it the 'core' in general, the right answer should be C.