Final answer:
The most susceptible portion of the humerus to fracture is the surgical neck, particularly following falls or direct impacts to the arm.
Correct option is C
Step-by-step explanation:
The portion of the humerus that is most susceptible to fracture is the surgical neck. The surgical neck of the humerus, located at the base of the expanded proximal end, where it joins the narrow shaft, is a common site for fractures, especially following a fall or a direct blow to the arm. Additionally, in children, a distal humerus fracture is common following a fall onto the tip of the elbow, which can lead to a supracondylar or intercondylar fracture.
Adult humerus fractures often occur at the surgical neck, while pediatric fractures often occur just superior to the epicondyles.