The correct answer is - a. a dense ball of solid metal.
Earth's inner core is the layer of the Earth that is the most dense, has the biggest mass, and is the one that mostly creates the gravitational pull of our planet. It is thought to be consisted of metal, solid one, but under enormous pressure and temperature. The metal that dominates in here is the iron, and it is accompanied by nikel. The inner core is essential for the life on Earth as it affects everything that happens on its surface. It is getting weaker over time, and once it is in function anymore the life on Earth with vanish, luckily this will be in the far future.
