You are answering a USMLE-style medical question. Question: A 40-year-old man comes to the physician because of shortness of breath, double vision, and fatigue for the past 4 weeks. He has no history of serious medical illness and takes no medications. Physical examination shows drooping of the eyelids bilaterally. He is unable to hold his arms up for longer than 3 minutes. A CT scan of the chest shows an anterior mediastinal mass with smooth contours. A photomicrograph of a specimen from the mass is shown. Which of the following immunologic processes normally occurs in the region indicated by the arrow? Options: A) B-cell maturation B) V(D)J recombination C) Thymocyte formation D) Negative selection Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.