The correct answer is A) If power-driven, both vessels must exchange passing signals when in sight and passing within half mile of each other. This is correct because the Inland Navigation Rules (Rule 34) state that when vessels in sight of one another are approaching a bend in a channel or fairway where other vessels may be obscured, they must sound the appropriate whistle signal (one prolonged blast followed by two short blasts) to indicate their intention to pass. Both vessels must then exchange these passing signals, even if they are not yet in sight of each other, as long as they are within half a mile. The other options are incorrect because B) would require passing signals even if the vessels are not in sight, C) would not require any further signals once the initial prolonged blast is answered, and D) would only apply if one vessel is the give-way vessel in a crossing situation, not a bend in the river.