Final answer:
The solid layer of the Earth comprised primarily of iron and nickel is the inner core, surrounded by the liquid outer core and above which is the mantle and crust. Therefore, the correct option is C.
Step-by-step explanation:
The solid layer of the earth that is made mostly of iron and nickel is the inner core. The inner core is a very dense area under incredible pressure located at the center of the Earth and is thought to be composed of an iron and nickel alloy. It is surrounded by the outer core, which is made of liquid iron and nickel and is believed to be responsible for generating the Earth's magnetic field. Above the outer core is the mantle, a massive layer of the earth, consisting of solid rock that can behave plastically under certain conditions. The outermost layer is the crust, a thin layer differentiated into continental and oceanic types, which are less dense than the mantle and core layers.