A 45-year-old woman presents with progressive proximal muscle weakness and ptosis. Investigations reveal serology positive for anti-acetylcholine receptor antibodies. A contrast-enhanced CT chest is performed to evaluate for an associated mediastinal mass. The structure marked **B** in the diagram is the most likely site of a pathological lesion in this patient. Which of the following is the most common neoplasm arising from the region marked **B** that is associated with this clinical presentation?