Concept:The one-way flow of water over the gills allows gaseous exchange in fish.Explanation:The fish first opens its mouth to draw in oxygen-rich water.The water moves from the mouth over the gill filaments, where oxygen is absorbed and carbon dioxide is released.Then the deoxygenated water is forced out through the operculum, the protective gill cover.Therefore, the correct one-way sequence is mouth → gills → operculum.Answer:Option B: mouth → gills → operculum.