Hint Protists are unicellular, eukaryotic microorganisms mostly found in water. They bear chlorophyll and photosynthetic algae and some are saprophytic like fungi. Parasitic protist lives on or inside the body of other organisms like in the digestive tract.
Multiple fission takes place in parasitic protozoans like Sporozoa. Sporozoans are very simple in structure without locomotor organs or contractile vacuoles. They may be histozoic or coelozoic. Reproduction takes place both sexually or asexually. In sexual reproduction, multiple fissions take place in which the nucleus divides repeatedly to produce several nuclei. A most common example of sporozoans is Plasmodium.
Under unfavorable conditions amoeba also shows multiple fission for survival.
Plasmodium causes malaria in humans which was first discovered by Ronald Ross in 1898. The life cycle of parasites is complete in two hosts like a mosquito and a man. This parasite does not harm mosquitoes but causes disease in humans. When a mosquito bites a healthy person, the parasites in the form of sporozoites enter the blood and reach the liver cells where it multiplies and reproduces asexually by multiple fission and the liver cell gets ruptured by liberating parasites into blood called merozoites. The merozoites then enter the RBC and then the RBC gets ruptured by causing the cycle of fevers and released parasites to start to infect new RBCs. The RBC gets attacked by macrophages and after a few generations sporozoites enter the RBC and grow into gametocytes, which are of two types microgamete and macrogamete. The Remaining process of reproduction takes place inside the mosquito when they bite an infected person. When a gametocyte enters the gut of a mosquito it loses its covering and undergoes maturation to form gametes. The fusion of male and female gametes produces a diploid zygote which becomes worm-like and produces 50-2000 oocytes which function as sporont. The sporont divides first meiotically and then mitotically to form a large number of haploid nuclei. The cytoplasm gathers around the nucleus and forms sporozoite, the cyst wall gets ruptured and passes into the hemocoel. Sporozoites enter the salivary gland for passage into new hosts.
So, the correct answer is Plasmodium.
Note: Plasmodium occurs in four different species and causes a different type of malaria. Plasmodium vivax causes Benign tertian malaria, Plasmodium falciparum causes tropical tertian malaria, Plasmodium malariae causes Quartan malaria and Plasmodium ovale causes mild tertian malaria.