The Earth is divided into four main layers: the solid crust on the outside, the mantle, the outer core, and the inner core. Explanation & Key Takeaways: The Core - • The earthquake wave velocities helped in understanding the existence of the core of the earth. • The core-mantle boundary is located at the depth of 2,900 km. • The outer core is in a liquid state while the inner core is in a solid state. • The density of material at the mantle core boundary is around 5 g/cm^(3) and at the centre of the earth at 6,300 km, the density value is approximately 13g/cm^(3). • The core is made up of very heavy material mainly constituted by nickel and iron. • It is sometimes referred to as the nife layer. The correct answer is option 4, i.e Nickel and Iron.