The correct answer is D) excessively early injection timing. Excessively early injection timing can result in high cylinder firing pressure and low exhaust temperature. This is because the fuel is injected before the optimal timing, causing the combustion process to occur earlier in the engine cycle. This leads to higher in-cylinder pressures and temperatures, but the heat is not fully utilized to produce power, resulting in lower exhaust temperatures. The other options are incorrect because: A) improper fuel rack positioning would affect fuel delivery but not necessarily cause the specific symptoms described; B) lengthy exhaust valve duration would affect the exhaust process but not necessarily the injection timing; and C) extended operation at light load would not directly cause the high pressure and low temperature issue.