You are answering a USMLE-style medical question. Question: A 54-year-old man comes to the clinic for management of his gastroesophageal reflux disease (GERD). He is currently on cimetidine for his condition and reports that his GERD initially improved. However, following his recent move to Michigan, he is experiencing more frequent episodes of chest pain (4-5 times a week). The pain is described as burning in quality and is concentrated around his epigastric region following food ingestion. It does not radiate anywhere and is alleviated when he takes antacids. A physical examination demonstrates a healthy male with unremarkable findings. He is subsequently prescribed a new medication for control of his symptoms. What is the most likely mechanism of action of this new medication? Options: A) Irreversible inactivation of cyclooxygenase enzyme B) Irreversible inhibition of H+/K+-ATPase at parietal cells C) Reversible inhibition of H+/K+-ATPase at parietal cells D) Reversible inhibition of histamine H2 receptors Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.