What part of the Earth’s structure is made mostly of iron and nickel?

The core of the Earth is primarily composed of iron and nickel. It is situated deep within the Earth, beneath the mantle and crust. The inner core, which is solid, is surrounded by a liquid outer core. The presence of these metals creates the conditions necessary for generating the Earth's magnetic field through the movement of molten metal in the outer core.

In contrast, the crust is the outermost layer of the Earth, made up mainly of lighter materials such as silicates, while the mantle, located directly beneath the crust, is composed of silicate minerals that are richer in magnesium and iron but not predominantly made of iron and nickel. The term "outer layer" is generally too vague, as it could refer to multiple layers and does not specify the core's composition. Thus, the core stands out as the correct answer due to its distinct composition of iron and nickel.