Final answer:
The scapula is the bone that forms a broad triangle and is part of the pectoral girdle. It plays a crucial role in the function of the shoulder joint alongside the clavicle, but the scapula itself is correctly described by the question.
Step-by-step explanation:
The bone described as forming a broad triangle is the scapula, also known as the shoulder blade. It is part of the pectoral girdle, which attaches each upper limb to the axial skeleton, together with the clavicle (collarbone). The scapula is a flat, triangular-shaped bone that has a complex structure which includes the spine of the scapula, the acromion, and the glenoid cavity, all of which play key roles in the articulation and function of the shoulder joint. Unlike the scapula, the clavicle is an S-shaped bone that serves as a strut extending laterally to support the scapula and holds the shoulder joint superiorly and laterally from the body trunk, providing maximum freedom for arm movement. The sternum is an elongated bony structure in the center of the chest, and the radius is one of the two bones of the forearm. Therefore, the scapula is the bone described in the question.