A mono-genetic parasite refers to a parasite that completes its entire life cycle on a single host species. Option 1, Hydra, is not a parasite but a multi-cellular organism found in freshwater environments. Option 2, Fasciola, is a parasitic flatworm that can infect various hosts, including humans and animals. Option 3, Ascaris, is a correct answer as it is a parasitic roundworm that infects humans and pigs, completing its entire life cycle within a single host. Option 4, Earthworm, is not a parasite but a beneficial decomposer in the soil. Therefore, the correct answer is option 3, Ascaris.