Final answer:
The elbow serves to A. position the hand, shorten and lengthen the arm, and participates in the complex action of forearm pronation and supination.
Step-by-step explanation:
The functions of the elbow in terms of movement of the upper extremity are indeed substantial and diverse. It positions the hand in space so it can perform a variety of activities, such as grasping or manipulating objects. In addition, the elbow shortens the arm when the forearm is flexed, which is important for bringing objects closer to the face, for instance when eating. Furthermore, when the forearm is extended, the elbow helps to lengthen the arm, enabling one to reach out for objects that are further away.
The movement mechanics of the elbow involve flexion and extension, which decrease and increase the angle at the joint respectively. This is aided by muscles like the biceps brachii and the brachialis. Additionally, the elbow joint is involved in the pronation and supination of the forearm. Supination refers to the rotation of the forearm so the palm faces upwards, while pronation is the rotation that turns the palm downwards.
Considering all these aspects, the correct answer to the question is "D. All of the above," as the elbow plays a crucial role in all these actions which are essential for the diverse range of motions we require from our upper extremities.