The correct option is D Helminths
Filariasis is caused by Wuchereria bancrofti (the filaria worm) that inhabit the lymphatic vessels. Wuchereria bancrofti is a thread-like filarial nematode (roundworm) and belongs to helminths. Viruses are acellular obligate parasite while bacteria are prokaryotes. Protozoans are eukaryotic unicellular organisms while Wuchereria is multicellular eukaryote with organ system level body organization. Thus, the correct answer is option D.