Solution: Rohu, Catla, and common carp are freshwater fish, while Hilsa is both a freshwater and a marine fish. Freshwater fish are hypertonic to their surroundings, meaning that the concentration of salt in their blood is higher than in their surroundings. Marine fish, in contrast to freshwater fish, have the opposite problem. They have a higher water concentration in their blood than the rest of the population. Hence, the correct answer is (b).