The correct answer to this question is A, A dense ball of solid metal. The inner core is the innermost section of Earth. The inner core is very hot and it is thought that it is just as hot as the surface of the Earth. Early on, the inner core was though to be liquid, but this was proved wrong in 1936.
Inge Lehmann, who is a seismologist, provided that the inner core was instead a solid, while the outer core was liquid. The belief is that the inner core is made of a metal and that metal is thought to be an iron-nickel alloy.