The answer to this is Mantle which is letter B in the choices. The mantle is located in between the crust of the earth and the outer core. The fact that it is rocky may contribute to its overall mass. This is known to have more mass than the other layers of the earth. The outer layer or the crust is known to be the thinnest layer.
The mantle is known to be very hot so even if people would be able to reach this far, the heat will be too much. There are different minerals that are found in this layer and they may change depending on their location and the pressure that is being experienced by the earth.