Rules of the Road: BOTH INTERNATIONAL & INLAND You are underway in a fog when you hear a whistle si...

35 (c) lists this signal be used for a vessel Not Under Command along with a vessel ENGAGED in towing or pushing another vessel. Not a vessel being towed. That situation is covered in 35 (e).

a vessle being towed sounds 1 prolonged blasts and 3 short blasts this one is wrong

Be careful and read the question (RFTQ as we say). The question is asking which one is NOT supposed to sound one prolonged and two short (note the "with the EXCEPTION" part of the question). You are right in saying that a vessel being towed (if manned) sounds one prolonged and 3 short, which is exactly why answer "D" is the correct answer, because it is not supposed to sound one prolonged and two short. RFTQ.