Concept: Water snails act as an intermediate host in the life cycle of the liver fluke.Explanation:The liver fluke is a parasitic flatworm that requires two hosts to complete its life cycle.The first host is a water snail, inside which the fluke passes through several larval stages.The second host is usually a mammal such as sheep or cattle, where the adult fluke matures and reproduces.Tapeworms and roundworms do not require a water snail as an intermediate host.Thus, the water-snail is important specifically for the liver fluke.Answer: D. liver fluke