Several deficits in muscle function of the right upper limb were noted, including inability to abduct the arm. This was caused by denervation of which muscle?
Although supraspinatus initiates abduction of the arm, the deltoid muscle allows the arm to continue abducting beyond the first 15 to 20 degrees. It is innervated by the axillary nerve, which must have been damaged in this injury. (Remember: the posterior circumflex aery had to be ligated, and it runs in the quadrangular space along with the axillary nerve. So, it's pretty conceivable that the axillary nerve has been injured, too.) Infraspinatus and teres minor are both lateral rotators of the arm. Since teres minor is also innervated by the axillary nerve, it might be injured, too, but the question isn't asking about this muscle. Latissimus dorsi is a medial rotator of the arm, innervated by the thoracodorsal nerve.
Deltoid
Infraspinatus
Latissimus dorsi
Teres minor