This disease is caused by a nematod

The correct option is Filariasis.

To understand why this is the correct answer, let us look at the causative agents of each of the diseases listed in the options:
1. Amoebiasis: This is an infection of the intestines caused by the protozoan parasite Entamoeba histolytica. Since it is caused by a protozoan and not a nematode, this option is incorrect.
2. Leprosy: Also known as Hansen's disease, this is a chronic infectious disease caused by the bacterium Mycobacterium leprae. Since it is bacterial, this option is incorrect.
3. Filariasis: Lymphatic filariasis (often known as elephantiasis) is caused by parasitic infection with nematodes (roundworms) of the family Filarioidea. These microscopic, thread-like worms, such as Wuchereria bancrofti, Brugia malayi, and Brugia timori, live in the lymphatic system of humans. Therefore, this disease is indeed caused by a nematode.
4. Poliomyelitis: Commonly known as polio, this is a highly infectious viral disease caused by the Poliovirus. Since it is caused by a virus, this option is incorrect.

Thus, filariasis is the only disease among the options that is caused by a nematode.