Contractile vacuoles keep this in check. They expand when water enters (called the diastole) and contract when filled with water, carrying water along with wastes outside of the cell (called the systole). The following image is a paramecium aurelia, a single-celled ciliated protozoa with two contractile vacuoles.

The correct answer is option 'D' - Plasmodium

Plasmodium is a protozoan that causes malaria in humans and other animals. It belongs to the phylum Apicomplexa and is transmitted through the bite of infected female Anopheles mosquitoes. Unlike the other protozoans mentioned in the options, Plasmodium does not possess a contractile vacuole.

Contractile vacuole in protozoans:
A contractile vacuole is a specialized organelle found in many freshwater protozoans. It is responsible for regulating water content and maintaining osmotic balance within the cell. The contractile vacuole collects excess water and expels it from the cell, preventing the cell from bursting due to osmotic pressure.

Protozoans with contractile vacuoles:
Out of the options given, Euglena, Paramecium, and Amoeba are known to possess contractile vacuoles.

- Euglena: Euglena is a unicellular organism that exhibits characteristics of both plants and animals. It has a flagellum that helps in locomotion, and it possesses a contractile vacuole to regulate its water balance.

- Paramecium: Paramecium is a ciliated protozoan that is commonly found in freshwater environments. It uses cilia for locomotion and also possesses a contractile vacuole to maintain osmotic balance.

- Amoeba: Amoeba is a single-celled organism that moves using pseudopodia, which are temporary extensions of its cell membrane. It also possesses a contractile vacuole to regulate water content and prevent excessive intake.

Plasmodium and absence of a contractile vacuole:
Plasmodium is a parasitic protozoan that infects the red blood cells of its host. Unlike the other protozoans mentioned, Plasmodium does not possess a contractile vacuole. This is because Plasmodium has evolved to live within the host's bloodstream, which has a relatively stable osmotic environment. Therefore, it does not require a contractile vacuole for water regulation.

Instead of a contractile vacuole, Plasmodium has other adaptations to survive within its host, such as the ability to evade the host immune system and reproduce within the red blood cells.

In conclusion, Plasmodium is the protozoan that is unlikely to have a contractile vacuole, as it has evolved to live within the host bloodstream and does not require osmoregulation through this organelle.

The biodiversity test here says amoeba is the right answer. But amoeba has a contractile vacuole.