Which motion would be classified as an extension?

Extension is a specific type of joint movement that increases the angle between two body parts, effectively straightening them. In the context of the options provided, straightening the leg exemplifies this movement since it involves increasing the angle at the knee joint.

When the leg is straightened, the knee moves from a flexed (bent) position to a more extended position, thereby increasing the angle between the thigh and the lower leg. This is precisely what defines an extension movement, making it the correct choice in this scenario.

In contrast, raising the arm to the side typically involves abduction rather than extension, bending the knee is classified as flexion because it decreases the angle at the joint, and rotating the hand pertains to a different type of movement known as rotation, which does not involve extensions or flexion at the joint.