Final answer:
Parasitic flatworms inhabit snail eyestalks to make them appear more appetizing to birds, which helps the flatworms complete their life cycle within the bird's digestive system.
Step-by-step explanation:
Parasitic flatworms travel into the eyestalks of their snail hosts to manipulate the snail's appearance and behavior, making the eyestalks appear more appetizing to birds. This is a survival strategy known as predator manipulation, where the parasite alters its host to increase its chances of being eaten by the predator, in this case, birds. Once the birds consume the infected eyestalks, the flatworms complete their life cycle within the bird's digestive system, where they can reproduce and then release their eggs.
The correct answer to the question is: D) The flatworms cause the snail’s eyestalks to look appetizing to birds.