The answer is C.
Layer C is the mantle of the earth and it is the layer immediately below the upper-most layer of the earth, the crust. The substances in the mantle are in the molten state due to the high temperature and the liquid form is called magma. Once a volcano erupts and the magma is released to the surface of the air, it is called lava.
Plants are living things because they are made of living cells and exchange gases with the air around.