Why does the core contribute greatly to Earth total mass?

Correct answer: Because it contains very dense metallic materials

The direct answer is A: the core contains very dense metallic materials. Earth’s mass depends mainly on how much matter it has and how densely that matter is packed. The core is made chiefly of iron and nickel, which are dense metals. Thus, although the core occupies a smaller volume than the mantle, its material contributes greatly to Earth’s total mass. B is wrong because being the thinnest layer would not itself explain a large mass contribution. C is wrong because the core is not gas; it has a solid inner part and a liquid outer part. D is wrong because ocean water is not the material of the core. The reasoning is: dense iron-nickel material → high density → considerable mass. Exam cue: mass is linked with density, not merely thickness.