The correct answer is C) Sound a whistle blast of 4 to 6 seconds duration. This is the correct answer because under the International and Inland Navigation Rules, when approaching a bend in a channel where the view is obstructed, vessels must sound a prolonged blast on the whistle to alert any other vessels that may be on the other side of the bend. This is a safety measure to warn other vessels of your presence and intended navigation around the blind bend. The other answer choices are incorrect because: A) Passing signals are not required in this situation, B) Stopping engines is not necessary if you can navigate cautiously, and D) Staying in the middle of the channel does not address the need to alert other vessels of your approach.