Final answer:
The trapezius, levator scapulae, and rhomboids are the three posterior muscles that act on the pectoral girdle, with the rhomboids being the correct answer to complete the mentioned group.
Step-by-step explanation:
The trapezius, levator scapulae, and rhomboids (rhomboid major and rhomboid minor) are the three muscles of the posterior group that act on the pectoral girdle. Among the choices provided, the correct answer is (d) rhomboid. When the rhomboids are contracted, the scapula moves medially, which can pull the shoulder and upper limb posteriorly. This action aids in the stabilization of the shoulder joint and is essential for the functionality of the upper limb.