Concept: The trochlea at the lower end of the humerus fits into the ulna to form the elbow hinge joint.
Explanation: The trochlea is the pulley-shaped projection found at the distal end of the humerus.
The upper end of the ulna carries a deep, C-shaped depression called the trochlear notch, which grips the trochlea.
This humero-ulnar connection acts like a door hinge; it allows only flexion and extension of the forearm.
The radius, on the other hand, mainly articulates with the rounded capitulum of the humerus, not with the trochlea.
Therefore, in the given diagram, the labelled part that articulates with the trochlea to form the hinge joint is the ulna.
Answer: B. II
