Which of the following is unsegmented?

Answer: Option C

Roundworm and Trematode are unsegmented.
Worms with round, non-segmented bodies are known as nematodes or roundworms. They are classified in the phylum Nematoda, which has over 28,000 known species.
Trematoda is obligate internal parasite with a complex life cycle requiring at least two hosts. These are unsegmented, leaf-shaped worms that are flattened dorsoventrally. They bear 2 suckers, one surrounding the mouth (oral sucker) and another on the ventral surface of the body (ventral sucker).