The correct answer is B) a mooring buoy. A white buoy with a blue band is a mooring buoy, which is used to mark a fixed location where a boat can be safely secured. Mooring buoys are typically found in harbors, marinas, and other areas where vessels may need to tie up. The other answer choices are incorrect because: A) an isolated danger mark has a black and white striped pattern, C) a restricted area is typically marked with a buoy that has red and white vertical stripes, and D) a hydrographic data collection buoy is usually marked with yellow or other colors, not a white buoy with a blue band.