Why does density generally increase toward Earth's centre?

Correct answer: Heavy elements concentrated below due to density differentiation

The direct answer is A. Earth became internally differentiated when it was much hotter and partly molten. Dense materials, especially iron and nickel, moved downward under gravity and became concentrated in the core, while lighter rocky materials remained above. This process is called density differentiation or gravitational differentiation. Therefore density generally rises from the crust toward the centre. Option A is correct because it gives the actual cause: heavy elements concentrated at depth. Option B is wrong because sunlight does not reach Earth’s centre and does not arrange the layers. Option C is wrong because rainfall cannot travel through the crust to the centre, and water is not the cause of the metallic core. Option D is wrong because the atmosphere is a very thin outer envelope and does not simply settle into the core. The core is denser than the mantle, and the mantle is generally denser than the crust. Memory cue: “heavy sinks, light stays above.”