Concept:An ecto-parasite lives on the outer surface of a host's body, while an endo-parasite lives inside the host.Explanation:Fleas attach to the skin or fur of animals and feed on their blood from outside.Therefore, fleas are ecto-parasites.Tapeworm, liver fluke, and roundworm live inside the host's body, so they are endo-parasites.Hence, the only ecto-parasite among the given organisms is the flea.Answer:A. Fleas