Answer: The correct answer for the blank is- a. change its position relative to other objects.
Perception of motion can be described as a phenomenon of inferring the direction and speed and of objects, which are moving in a visual scene.
An object is said to be in motion if its position is changed with respect to objects that are present in its surrounding.
Thus, in order to sense motion of objects, humans beings are required to see an object change its position with respect to other objects.
