Explanation:
The correct option is 'a'. In pediatric supracondylar humerus fractures, the direction of distal fragment displacement dictates the structures at risk. With posteromedial displacement, the proximal fragment is driven anterolaterally, commonly impaling or stretching the radial nerve. Conversely, posterolateral displacement puts the median nerve and the anterior interosseous nerve (AIN) at higher risk. Overall, AIN neuropraxia is the most common nerve palsy in supracondylar fractures generally, but specifically in posteromedial displacement, the radial nerve is predominantly at risk.

Reference: Omokawa S, et al. Neurological complications in supracondylar fractures of the humerus in children. J Bone Joint Surg Am. 1998;80(10):1495-502.