The correct answer is C) that has a sharp drop off into deep water. A bluff bar is a bar formation that extends out from a bluff or cliff-like shoreline and has a sharp drop-off into deep water on the downstream side. This sudden change in depth can be hazardous to navigation, as it can create strong currents and turbulence. The sharp drop-off is the defining characteristic of a bluff bar, which makes answer C the correct choice. The other answer choices are incorrect because: A) a bar perpendicular to the current is not the defining feature of a bluff bar, B) a bluff bar extends out from a bluff but is not necessarily alongside the river, and D) a bluff bar does not necessarily give a false indication of its position, but rather the sudden change in depth is the key hazard.