The layer of Earth composed primarily of solid iron is the inner core.

Here's why:

* Earth's Structure: Earth has several layers: crust, mantle, outer core, and inner core.

* Inner Core Composition: The inner core is primarily made up of iron (with a small percentage of nickel and other elements). The immense pressure at the Earth's center keeps the iron in a solid state, even though its temperature is incredibly hot.

* Outer Core Composition: The outer core is also primarily made up of iron and nickel, but it's in a liquid state due to the lower pressure.

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