You are correct, the answer is (B) reduce engine load. If the jacket water temperature rises rapidly above normal in a diesel engine, the first step should be to reduce the engine load. This is because a rapid rise in jacket water temperature indicates a problem with the engine's cooling system, which could lead to engine damage if not addressed promptly. Reducing the engine load will help to lower the engine's temperature and prevent further overheating. The other options are not the correct first step. Cleaning the sea water strainer (A) may help in the long run, but it does not address the immediate issue of the rising temperature. Calling the chief engineer (C) is not the first step, as the operator should try to address the problem themselves before involving higher-level personnel.