* Force and Motion: Forces cause changes in an object's motion. This can be a change in speed (acceleration or deceleration) or a change in direction.

* Perpendicular Force: When a force acts perpendicular to an object's motion, it only changes the object's direction. Imagine pushing a ball sideways while it's rolling forward. The ball will change direction but its speed will remain the same.

* Other Options:

* A force acting parallel to the object's motion: This would change the speed of the object (acceleration or deceleration) but not its direction.

* No force acting on the object: The object would continue moving at a constant speed and in a straight line (Newton's First Law of Motion).

* A force acting at an angle to the object's motion: This would change both the speed and direction of the object.

In summary: A force acting perpendicular to an object's motion is the only option that changes direction without affecting speed.