Concept:Ascaris lumbricoides belongs to the group Nematoda, which are commonly called roundworms.
Explanation:Ascaris lumbricoides is a parasitic worm that lives in the human intestine.
It has a long, cylindrical, unsegmented body, which is a characteristic feature of roundworms.
Flatworms are flattened and usually segmented or leaf-like, so they are not the correct group.
Arthropods have jointed legs and an exoskeleton, which does not match this organism.
There is no biological group called elastic worm.
Therefore, Ascaris lumbricoides is correctly identified as a roundworm.
Answer:B. round worm