Which is a helminth disease

The correct option/answer is Filariasis.

To understand why this is the correct answer, let's break down the nature of the diseases listed in the options:
1. Polio: Short for poliomyelitis, this is a highly infectious disease caused by a virus (the poliovirus).
2. Filariasis: This is a parasitic disease caused by microscopic, thread-like roundworms belonging to the superfamily Filarioidea. These worms are helminths (parasitic worms). Therefore, filariasis is classified as a helminth disease.
3. Filaria: This refers to the parasitic worm itself (the causative agent) rather than the disease name.
4. Diphtheria: This is a serious infection caused by strains of bacteria called Corynebacterium diphtheriae that make toxin.

Since helminths are parasitic worms, and filariasis is a disease caused by infection with these filarial roundworms, it is classified as a helminth disease.