Final answer:
The radial notch of the ulna is where the head of the radius articulates with the ulna, forming the proximal radioulnar joint which allows for forearm rotation.
Step-by-step explanation:
The region of the radius that articulates with the ulna is the radial notch of the ulna at the proximal radioulnar joint. This articulation allows for the rotation of the forearm. The head of the radius fits into the radial notch of the ulna, enabling this joint's movement. The region of the radius that articulates with the ulna is the head of the radius.
The head of the radius forms the proximal end and has a small depression on its surface that articulates with the capitulum of the humerus as part of the elbow joint. The smooth, outer margin of the head articulates with the radial notch of the ulna at the proximal radioulnar joint.