Which layer forms the largest portion of earth?

The correct answer is mantle.

Explanation:

The Earth is formed of four distinct layers. The crust refers to the layer, on which we live, the mantle is much hotter and possesses the tendency to flow, and the inner core and the outer core. The crust is the thinnest layer in comparison to the other three layers, that is, it is only about 3 to 5 miles thick underneath the oceans, and about 25 miles thick underneath the continents.  

Mantle refers to the layer, which is situated directly underneath the sima, that is, the lower layer of the Earth's crust and is the largest layer of the planet, that is, about 1800 miles in thickness. This section of the Earth comprises very hot and dense rocks. The temperature of the mantle differs from 1600 degrees F at the top to about 4000 degrees F close to the bottom.  

it would be c. xylem.