According to perception of movement, in order to sense movement, humans need to see an object

a. change its position relative to other objects

b. change its position, regardless of what's around it

c. change its position relative only to other moving objects

d. change its position while something else moves in the same way

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.

it's a b and