You are answering a USMLE-style medical question. Question: A healthy 21-year-old man undergoes physical fitness testing prior to long-term submarine deployment. To evaluate his pulmonary function, lung and thoracic compliance are measured at different system pressures. A graph of the relationship between his lung volume and transpulmonary pressure is shown. The dotted line in this graph corresponds to which of the following lung volumes? Options: A) Inspiratory capacity B) Tidal volume C) Functional residual capacity D) Total lung capacity Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.