The correct answer is D) Reduce engine speed. The reasoning behind this is that a high lube oil sump temperature alarm indicates an issue with the engine's lubrication system, likely excessive friction or lack of cooling. Reducing the engine speed will help to lower the load on the engine and reduce the heat generation, which can help to lower the oil sump temperature and prevent further damage. This is a recommended first step in response to a high oil sump temperature alarm, as per standard operating procedures for gas turbine engines. The other options are not the appropriate first step in this scenario. Checking the oil filter differential pressure (A) or the oil pressure to the sump (C) may provide additional information, but would not directly address the high temperature issue. Decoupling the engine from the main reduction gear (B) would be a more drastic measure that should only be considered if the high temperature persists after reducing engine speed.