The correct option is D Caenorhabditis
Caenorhabditis is a free-living round worm whereas Ascaris, Ancylostoma and Wuchereria are parasitic round worms.
Ascaris and Ancylostoma (Hookworm) causes an infection of the intestines known as Ascariasis and Ancylostomiasis, respectively.
Wuchereria causes a lymphatic infection known as lymphatic filariasis or elephantiasis.