The answer is d. Protist.
Protists are the causes of Malaria, African sleeping sickness and amoebic dysentery.
It is a large group of single cell microorganism that does not form into tissues. Protists may reproduce sexually through gametes and asexually through binary fission. Protists may consider as invertebrate parasites. They were divided into different groups;
=> Protozoa
=> Protophyta
=> Slime molds
=> water molds
Protists mostly lives in water, any kind of water. Algae is one example of protists