Final answer:
The prime mover for scapular retraction is the rhomboid muscle, which when contracted, moves the scapula medially and retracts the shoulder.
Step-by-step explanation:
The prime mover for scapular retraction is the rhomboid muscle. The rhomboids consist of the rhomboid major and rhomboid minor muscles, which are located on the posterior thorax. When these muscles contract, they move the scapula medially towards the vertebral column. This action retracts the scapula, pulling the shoulders and upper limb backward. Therefore, the correct answer to the question is b. Rhomboid.
Other muscles mentioned, such as the serratus anterior and pectoralis minor, are located on the anterior thoracic wall and are involved in different movements of the scapula and pectoral girdle. The coracobrachialis is a muscle of the arm that assists in flexing and adducting the shoulder, not in scapular retraction.