Which of the following protozoans show bioluminescence?

The correct option is Noctiluca.

Bioluminescence is the biochemical phenomenon where a living organism produces and emits light. This light emission is the result of a chemical reaction where a light-emitting molecule, luciferin, is oxidized in the presence of the enzyme luciferase.
Among the options provided, Noctiluca (specifically Noctiluca scintillans, commonly referred to as the "sea sparkle") is a free-living marine dinoflagellate protozoan that exhibits bioluminescence. When disturbed by mechanical wave action or predators, these organisms produce a bright blue flash of light, creating glowing patterns in the ocean.

Here is why the other options do not show bioluminescence:
1. Opalina: This is a genus of multinucleated, flagellated protozoans that live as commensals inside the intestines of amphibians (such as frogs and toads). They do not produce light.
2. Entamoeba: This is a genus of amoeboid protozoans. Many species, such as Entamoeba histolytica, are parasites that cause diseases like amoebiasis in humans. They live in dark host environments and do not exhibit bioluminescence.
3. Paramecium: This is a well-known genus of unicellular ciliated protozoans found widely in freshwater habitats. They are simple, free-living organisms that lack the chemical pathways required to produce bioluminescence.