Which of the following is best demonstrated on a lateral projection of the elbow?

The lateral projection of the elbow is designed to provide a clear view of the olecranon and its associated olecranon process. This positioning showcases the anatomy in such a way that the olecranon, which is the bony prominence of the ulna at the elbow, is prominently displayed. The lateral view allows for a better understanding of the anatomy and alignment of the ulna relative to the humerus. When the elbow is positioned laterally, the olecranon process sits in profile, making it easier to identify any abnormalities or fractures that may affect this area. Thus, it is the best demonstrated structure in this particular projection.

The other options, while relevant to elbow anatomy, do not show as clearly in a lateral view. The capitulum and trochlea are best visualized in an anterior-posterior view. Radius and ulna alignment can be assessed in an anteroposterior or axial view, and the medial and lateral epicondyles are also more distinctly seen in a frontal projection, not laterally.