Step 1: Read the question.
We must choose the correct statement about the lithosphere.
Step 2: Earth's spheres.
Earth is made of several layers or spheres: the lithosphere (solid land), hydrosphere (water), atmosphere (air), and biosphere (life).
Step 3: What the lithosphere is.
The lithosphere is the hard, rocky outer shell of the Earth. It is made of the crust and the topmost solid part of the mantle.
Step 4: What it covers.
It includes continents, mountains, plateaus, and even the ocean floor. So it is the solid crust of the Earth, not just one part of it.
Step 5: Rule out the wrong choices.
It is not only the ocean floor, not only soil, and it has nothing to do with air layers. Those statements are false.
Step 6: Conclude.
The correct statement is that the lithosphere is the solid crust of the Earth.
\[ \boxed{\text{It is the solid crust of Earth.}} \]