The correct answer is C) overspeed trip. Sudden unloading of a diesel engine, such as when the load is suddenly removed, can cause the engine to overspeed. This sudden loss of load can lead to a rapid increase in engine speed, which can trigger the engine's overspeed protection mechanism, known as an overspeed trip. The overspeed trip is a safety feature designed to shut down the engine and prevent damage from excessive speeds. The other answer choices are incorrect because they do not directly address the issue of sudden unloading leading to an overspeed trip. Decreased fuel efficiency (A), increased exhaust temperature (B), and black smoke (D) may be symptoms of other engine issues, but they are not the primary consequence of sudden unloading.