This option is correct because all the diseases are caused by protozoa.
Balantidiasis is a protozoan infection caused by Balantidium coli.
Malaria parasites belong to the genus Plasmodium (protozoan).
Kala-azar (leishmaniasis) is caused by Leishmania donovani (protozoan).
Giardiasis is caused by microscopic protozoan parasites known as Giardia intestinalis.