Scientists believe that differences in cause hot, plastic-like rock in the asthenosphere to rise toward earth's surface.
a. density
b. magnetism
c. weight
d. composition

The correct option is A.
The asthenosphere refers to the section of the earth mantle that is lying beneath the lithosphere. Heat that originates from within the earth crust keep the asthenosphere malleable and allow for the movement of tectonic plates. In accordance with plate tectonic theory, the asthenosphere contains the old and the denser parts of the lithosphere, which are usually dragged downward in subduction zones while the less dense plastic like rocks rise toward the earth surface.   

Scientists believe that differences in density cause hot, plastic-like rock in the asthenosphere to rise toward the Earth's surface. The correct answer will be A.