Final answer:
The Subclavius muscle originates from the first rib and functions to lower and move the clavicle downward.
Step-by-step explanation:
The muscle of the thorax that has an origin on the first rib and functions to lower and move the clavicle is C) Subclavius.
This muscle is one of the anterior thoracic muscles, which include the subclavius, pectoralis minor, and serratus anterior. The subclavius muscle is located underneath the clavicle, and when it contracts, it depresses the clavicle, stabilizing it during movements of the pectoral girdle and upper limb. The other options provided differ in their functions and origin; the pectoralis major primarily moves the humerus, the serratus anterior facilitates movements of the scapula, and the latissimus dorsi primarily acts on the humerus as well.
It is notable that the anterior thoracic muscles are important in the stabilizing and movement of the pectoral girdle which includes the clavicle and scapula.