The scenario described, where the student pushes the two halves of a hardcover book toward each other, models the process of mountain building (A). This is akin to the convergence of tectonic plates, where two plates collide, leading to the uplift of land and the formation of mountains.
Divergent plate boundaries (D) would involve moving the halves away from each other, which is not what is happening in this scenario.
Therefore, the best answer is A: mountain building.