Final answer:
Pi is not alerted by a specific event like a loud siren or sound of crashing waves, but most likely a combination of cues that indicate the ship is listing.
Step-by-step explanation:
The moment that alerts Pi to the fact that the ship is listing is not directly provided in the listed options a), b), or c). However, based on the context given and the usual indicators of a ship in distress, it can be determined that Pi might have been alerted by a combination of sudden movement of the ship, indicative noises such as crashing or a change in engine noise, or even a physical sensation of the ship tilting. In literature, particularly in survival narratives, the listing of a ship often creates dramatic tension and calls attention to the dire circumstances that characters must face. Therefore, the correct answer is d) None of the above.