Answer:
D. Crust
The crust is the outer layer of the earth. This is where plants and animals live. No one has explored any layer deeper than the crust in person.
The mantle, which is located under the crust, is made up of hot magma and rocks.
The outer core is made of liquid iron and nickel. It's the only layer made of complete liquid. It surrounds the inner core.
The inner core is at the very center of the Earth. It's a dense ball of iron.
Final answer: crust