Answer:
The correct answer is Nematoda.
Explanation:
This kind of worms belong to the:
The Dirofilaria immitis or also known as Hearthworm is a parasitic roundworm spread from host to host through mosquito bite, they have, as mentioned in the excerpt from the question, a pseudocoelom, an alimentary canal an unsegmented body and an outer covering, therefore this kind of worms is classified the Nematoida clade.