The right answer is (D) Continents. Plate tectonics is the scientific theory that the Earth's outer layer, the lithosphere, is broken up into large and small pieces called tectonic plates. These plates move on top of a layer of partially melted material called the asthenosphere. Plate tectonics is the latest scientific theory that incorporates and explains the former scientific theory of continental drift—the theory that the continents move around the Earth. Plate tectonics explains how the continents move, how mountains are formed, and why there are earthquakes and volcanoes by demonstrating how these large plates move.