The correct answer is C) engine overload. High exhaust temperature and black smoke exhaust from an auxiliary diesel engine are typically caused by engine overload. When the engine is operating under excessive load, it has to work harder to generate the required power. This leads to incomplete combustion, resulting in higher exhaust temperatures and the production of black smoke, which is a sign of unburnt fuel. The other options are incorrect because: A) A plugged fuel nozzle would lead to poor fuel atomization and incomplete combustion, but not necessarily high exhaust temperature and black smoke. B) Excessive compression pressure would generally result in higher exhaust temperatures, but not necessarily black smoke. D) Low combustion temperature would typically lead to incomplete combustion and black smoke, but not necessarily high exhaust temperature.