7. A rightward force is applied to a book in order to move it across a desk at
constant velocity. Consider frictional forces. Neglect air resistance. Diagram the
forces acting on the book.

The forces acting on the book on the horizontal are the moving force (force that causes pull or push on the body)

The frictional force Ff is also acting on the body. This is the force acting opposite the moving force "F"

Along the vertical component, the forces acting along with the component are the weight of the object and the reaction force acting opposite the weight.

Learn more here: link

See the explanation below

Explanation:

In the attached image we can see the free body diagram and forces acting on the book.

Force F, which acts to the right, the friction force which acts in the opposite direction to the movement, i.e. to the left.

The force exerted by the weight of the book that is equal to the product of the mass of the book by the gravity of the book. The normal force that is equal in magnitude to the weight of the book but in the opposite direction.

wavelength variability is the right one

there are no correct statementd on the list of choices you provided.