BOTH INTERNATIONAL & INLAND You are underway in heavy fog. You hear the fog signal of a vessel which is somewhere ahead of your vessel. You must __________.

Under Inland Navigation Rules, for the condition where 'You are underway in heavy fog. You hear the fog signal of a vessel which is somewhere ahead of your vessel. You must', the correct requirement is: 'slow to bare steerageway and navigate with caution'. This standard ensures clear navigational intent and collision avoidance.