Protista includes
(a)Unicellular prokaryotes
(b)Unicellular eukaryotes
(c)Bacteriophages
(d)Blue-green algae

Hint: It is unicellular in nature but can also be found as a colony of cells. Most of these survive in water, damp terrestrial environments, or even as parasites. These are usually aquatic, present within the soil or in areas with moisture.

Complete answer
A protist is a eukaryotic organism that is not an animal, plant, or fungus. They are eukaryotic unicellular organisms. They are grouped under a different kingdom by Whittaker in his five-kingdom classification.

Additional information:
-Most protist species are unicellular organisms, however, there are a couple of multicellular protists like kelp. Some species of kelp grow so large that they exceed over 100 feet tall. (Giant Kelp).
-Just like all other eukaryotes, the cells of those species have a nucleus and membrane-bound organelles.
-They may be autotrophic or heterotrophic in nature. An autotrophic organism can create its own food and survive. A heterotrophic organism, on the opposite hand, has got to derive nutrition from other organisms like plants or animals to survive.
-Symbiosis is observed within the members of this class. For instance, kelp (seaweed) may be a multicellular protist that gives otters protection from predators amidst its thick kelp. In turn, the otters eat sea urchins that tend to prey on kelp.

So the correct answer is ‘Unicellular eukaryotes’.

Note: Parasitism is also observed in protists. Species like Trypanosoma protozoa can cause sleeping sickness in humans.
Protists exhibit locomotion through cilia and flagella. A few organisms belonging to the dominion Protists have pseudopodia that help them to maneuver.
Protista reproduces by asexual means. It happens only during times of stress and the sexual method of reproduction is extremely rare.