Questions: Which statement best describes the theory of plate tectonics? Plates sometimes move closer to one another. Sections of Earth that shift are part of Earth's asthenosphere. Sections of Earth's continents are in slow, constant movement. Parts of the ocean floor move as they are carried by rift valleys.

The answer is the third one: Sections of Earth's continents are in slow, constant movement.

Explanation for each option:

1. Plates sometimes move closer to one another.

   • This statement is partially correct but incomplete. It describes convergent boundaries where tectonic plates move towards each other, but it does not encompass the full scope of plate tectonics, which also includes divergent and transform boundaries.

2. Sections of Earth that shift are part of Earth's asthenosphere.

   • This statement is incorrect. The asthenosphere is the semi-fluid layer upon which tectonic plates (part of the lithosphere) float and move. The shifting sections are part of the lithosphere, not the asthenosphere.

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

   • This statement best describes the theory of plate tectonics. It accurately reflects the idea that the Earth's lithosphere is divided into tectonic plates that are in continuous, slow motion, driven by forces such as mantle convection, slab pull, and ridge push.

4. Parts of the ocean floor move as they are carried by rift valleys.

   • This statement is somewhat misleading. While it is true that new oceanic crust is formed at mid-ocean ridges (which can be associated with rift valleys), the movement of the ocean floor is a result of the overall movement of tectonic plates, not just the rift valleys.

Summary: The theory of plate tectonics is best described by the statement that sections of Earth's continents are in slow, constant movement. This encompasses the continuous and dynamic nature of tectonic plate interactions that shape the Earth's surface.