The correct answer is B.
According to the viewpoint of Loki, he believes that whatever forces acting on the book are in balanced condition.
So, there is no such motion of the book possible in the horizontal direction.
BUT,
Remember this flowchart discussed in Newton’s first law?
If Loki has understood Newton’s laws properly, then he knows that…
If the friction and air resistance are neglected, there is no such kind of air drag, right.
So, the object starts moving in the horizontal direction. (Even if the forces are balanced)
Therefore, Sara is correct.
Always Remember,
An external force is not needed to produce the motion of any object.
External force only helps in accelerating the object forward.
(Object will not change its behaviour if no external force is given to it)