The BEST example of inertia from the given options is:
C A rock is thrown vertically up in the air and it falls back to the earth.
Inertia refers to the tendency of an object to resist changes in its state of motion. In this case, the rock's upward motion and eventual return to the ground illustrates how it continues in its state of motion until acted upon by gravity (which pulls it back down). The other options do not highlight inertia as effectively as this one does.