The correct answer is C) Sound a prolonged blast. The reason this is the correct answer is that according to both the International and Inland Navigation Rules, when a vessel is approaching a bend or an area of a channel or fairway where other vessels may be obscured by an obstruction, the vessel must sound a prolonged blast to alert any other vessels in the vicinity of its presence and approach. This is a standard navigation safety procedure to ensure that all vessels are aware of each other's positions and can take appropriate action to avoid a collision. The other answer choices are incorrect because: A) Sounding the danger signal is not the appropriate action in this situation; B) Taking all way off and navigating with caution is a good practice, but does not fulfill the requirement to sound a signal; and D) Posting a lookout is also a good practice, but does not replace the need to sound a signal.