BOTH INTERNATIONAL & INLAND You are underway in fog when you hear the following signal: one short blast, one prolonged blast and one short blast in succession. Which of the following would it be?

Under Inland Navigation Rules, for the condition where 'You are underway in fog when you hear the following signal: one short blast, one prolonged blast and one short blast in succession. Which of the following would it be', the correct requirement is: 'A vessel at anchor'. This standard ensures clear navigational intent and collision avoidance.