BOTH INTERNATIONAL & INLAND While underway in fog, you hear a vessel sound four short blasts in succession. What does this signal indicate?

Correct Answer: Option D — A pilot vessel on duty

Under Inland Navigation Rules, for the condition where 'While underway in fog, you hear a vessel sound four short blasts in succession. What does this signal indicate', the correct requirement is: 'A pilot vessel'. This standard ensures clear navigational intent and collision avoidance.