In the structure of planet Earth, below the mantle, the core is mainly made up of which one of the following?

Answer: (c) Iron

The core is mostly iron (with nickel) — NiFe.

The idea

During early Earth history, gravity helped separate materials by density. Dense metal moved towards the centre, while lighter silicate material stayed above it. The core and mantle therefore differ in composition as well as depth.

Early Earth heats up → Much material becomes able to flow → Dense metal sinks → Lighter silicates remain above → A metal-rich core and rocky mantle develop

Why the others are wrong

(a) Aluminium is light — it is in the crust.

(d) Silicon dominates the crust and mantle.

Elimination technique

Density separation points to a metal-rich centre. Aluminium and silicon are better associated with the rocky outer layers; among the metal options, iron is the dominant core element, not chromium.

Also know

The outer core is liquid and the inner core is solid because pressure as well as temperature controls melting. Movement in the conducting outer core helps generate the magnetic field. The Moho separates crust from mantle; the core-mantle boundary lies much deeper. The mantle is mostly solid rock, though it can deform slowly.