Final answer:
The movement that causes a decrease in the angle between bones is called flexion. Flexion involves a bending motion at a joint, which contrasts with extension that increases the angle at the joint.
Step-by-step explanation:
The type of movement that causes the angle between bones to decrease is known as flexion. During flexion, there is a bending motion that decreases the angle between the bones at a joint, such as when moving the forearm upward at the elbow. An example of flexion in another part of the body is dorsiflexion at the ankle, where the toes are lifted towards the knee. The opposite movement of flexion is extension, which increases the angle between the joints.
Moreover, the movements of joints in the human body can be classified into several types, including angular movements like flexion and extension, but also rotational movements, gliding movements, and special movements such as inversion and eversion. However, when addressing the angle between bones specifically, flexion is the correct answer as it fulfills the criteria of decreasing the angle at the joint.
Therefore, the answer to the student's question would be choice (b) - The angle between bones is decreased, as this defines what occurs during the flexion movement.