Final answer:
Abduction of the shoulder would be the most painful movement if there were tendinitis of the supraspinatus muscle, as this movement involves the supraspinatus tendon, which would be inflamed. The correct option is A.
Step-by-step explanation:
The movement of the upper extremity that would be most painful if there were tendinitis of the supraspinatus muscle is abduction of the shoulder. This is because the supraspinatus muscle is one of the muscles that forms the rotator cuff and is primarily responsible for initiating the abduction of the upper limb.
During superior rotation of the scapula, a necessary motion for full abduction of the upper limb, the supraspinatus tendon can become pinched and inflamed if tendinitis is present, causing significant pain.
Therefore, the correct option is A.