Which of the Earth's layers is the thinnest (only three to thirty miles thick) ?

a. the mantle

b. the outer core

c. the inner core

d. the crust

This is the crust - the thinnest and the outermost layer of the Earth!

The Crust is solid (the only other solid layer is the inner core) and it's the surface on which we walk. It's very thin under the Oceans - as little as 3 miles - but a lot more thick under the continents - up to 30 miles thick.