The correct answer is A) Allow the engine to cool gradually. When an engine shuts down due to high jacket water temperature, the best course of action is to allow the engine to cool gradually. This helps prevent thermal shock and potential damage to the engine components. Rapidly cooling a hot engine can cause warping or cracking of the engine block, cylinder heads, or other critical parts. The other options are incorrect because: B) Slowly adding cool water to the expansion tank could cause thermal shock; C) Opening crankcase explosion covers is not the appropriate response to a high temperature issue; and D) Back flushing the cooling system should only be done after the engine has cooled, not during the overheating incident.