The correct answer for this question is option 'C' - glenoid cavity. Let's understand why the humerus articulates with the glenoid cavity.

Head of the Humerus:
The humerus is the long bone that forms the upper arm. It consists of a rounded upper end called the head, which is located proximally.

Articulation with the Glenoid Cavity:
The glenoid cavity is a shallow, concave socket located on the lateral side of the scapula. It forms the glenohumeral joint, commonly known as the shoulder joint. The head of the humerus articulates with the glenoid cavity to form this joint.

Explanation:
When the rounded head of the humerus articulates with the glenoid cavity, it allows for a wide range of movement in the shoulder joint. The shallow nature of the glenoid cavity provides stability to the joint while allowing for flexibility and mobility.

The articulation between the humerus and the glenoid cavity is a synovial joint, which means it is surrounded by a joint capsule filled with synovial fluid. This fluid lubricates the joint, reducing friction and allowing for smooth movement.

Importance of the Glenohumeral Joint:
The glenohumeral joint is one of the most mobile joints in the human body. It allows for movements such as flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction of the arm. This joint plays a crucial role in activities that involve reaching, lifting, throwing, and other arm movements.

Conclusion:
In summary, the rounded upper end (head) of the humerus articulates with the glenoid cavity of the scapula to form the glenohumeral joint. This joint provides stability and allows for a wide range of movements in the shoulder.