Question

which statement best describes the theory of plate tectonics?
plates sometimes move closer to one another.
sections of earth that shift are part of earths asthenosphere.
sections of earths continents are in slow, constant movement.
parts of the ocean floor move as they are carried by rift valleys.

Explanation:

The theory of plate tectonics states that the Earth's lithosphere (which includes the continents) is divided into plates that are in slow, continuous motion. Plate interactions (convergent, divergent, transform) are constant, not just "sometimes" (eliminating "Plates sometimes move closer to one another"). The shifting sections are part of the lithosphere, not the asthenosphere (so "Sections of Earth that shift are part of Earth's asthenosphere" is wrong). Rift valleys are features formed by plate movement (divergent boundaries), not what "carries" ocean - floor parts (eliminating "Parts of the ocean floor move as they are carried by rift valleys").

Answer:

Sections of Earth's continents are in slow, constant movement.