Plate tectonics suggests that the ( a. biosphere) floats and moves on the ( a. atmosphere)
or
plate tectonics suggests that the ( b. asthenosphere) floats and moves on the ( b. lithosphere)
or
plate tectonics suggests that the ( c. geosphere) floats and moves on the ( c. asthenosphere)
or
plate tectonics suggests that the ( d. lithosphere ) floats away and moves on the ( d. geosphere)
which one ?

The right alternative is "plate tectonics suggests that the (b. asthenosphere) floats and moves on the  (b. lithosphere  )"

Here is a quick explanation:

Plate Tectonics is a theory of geology that describes the large-scale movements that occur in the Earth's lithosphere.   In the theory of plate tectonics, the outermost part of the Earth is composed of two layers: the lithosphere, which includes the crust and the solidified zone in the outermost part of the mantle, and the asthenosphere, which includes the innermost and viscous part of the mantle On a time scale of millions of years, the mantle seems to behave like a superheated liquid, but in response to sudden forces, like the earthquakes, behaves like a rigid solid. The lithosphere is divided in several tectonic plates and these moving over the asthenosphere .