Final answer:
The term 'shoulder girdle' informally refers to the pectoral girdle, which includes both the clavicles and scapulas. It is not exclusively one of the listed joints but the collective structure attaching the arms to the body, which includes the glenohumeral joint as the shoulder joint.
Step-by-step explanation:
The informal term "shoulder girdle" refers to the pectoral girdle, which is composed of the paired clavicles (collar bones) and the scapulas (shoulder blades). This structure serves to attach the arms to the trunk of the body. Although 'shoulder girdle' could be confused with a specific joint, the term is more accurately defined as including both the clavicle and the scapula. The shoulder joint itself, known for its wide range of motion, is termed the glenohumeral joint, which is a ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula.
Among the options given, A. Glenohumeral joint refers to the shoulder joint itself, B. Costoclavicular joint is a ligamentous structure, C. Acromioclavicular joint is where the scapula meets the clavicle at the top of the shoulder, and D. Sternoclavicular joint is where the sternum meets the clavicle. Therefore, the term 'shoulder girdle' encompasses both joints and bones of the upper shoulder region including the clavicle and scapula, rather than any single listed joint.