A 20-year-old man comes to the physician because of ...
Area labeled ‘C’
because raising the arm above the shoulder suggests abduction which would mean the axillary nerve is also involved. the lesion would be more proximal.
I thought "up to the shoulder" is done by deltoid muscle (aka axillary nerve) and above is done by trapezius.
as far as i can find, abduction ranges of motion, per first aid are:
0-15° = supraspinatus
15-90° = Deltoid
90° = trapezius
100° (over the head) = serratus anterior
in this question, they are directly telling you its not the serratus (long thoracic), because no option compromises it. Also, trapezius is innervated by cranial nerve XI, which is not a part of the brachial plexus, so, even though its worded weirdly, you can assume they are talking about deltoid disfunction.
so deltoid disfunction (axillary) + radial disfunction = posterior cord