What part of the ulna contacts the humerus during extension?

The olecranon process of the ulna is the correct answer because it is the bony prominence at the upper end of the ulna. During the extension of the elbow, this process fits into the olecranon fossa of the humerus, effectively locking the elbow joint in a straightened position. This bony configuration allows for stability and support during extension, making it a crucial part of the elbow joint's anatomy that facilitates movement and mechanical advantage when raising the forearm.

In contrast, the coronoid process, while also part of the ulna, is engaged during flexion of the elbow and does not contact the humerus during extension. The radial notch is a site that accommodates the radial head and is not involved in the extension motion. The head of the ulna is located at the distal end and does not articulate directly with the humerus, thus playing no role in the extension of the elbow joint. Understanding these anatomical relationships is vital for grasping how the elbow functions during different types of movement.