INLAND ONLY While underway in a harbor you hear a vessel sound a prolonged blast. This signal indicates that this vessel __________.

Correct Answer: Option A — is moving from a dock

Under Inland Navigation Rules, for the condition where 'While underway in a harbor you hear a vessel sound a prolonged blast. This signal indicates that this vessel', the correct requirement is: 'is moving from a dock'. This standard ensures clear navigational intent and collision avoidance.