The correct answer is option 3: Protozoan.
Option 1: Bacterium - Bacteria are single-celled organisms, but they do not cause malaria. Bacteria can cause various other infections, but malaria is not caused by bacteria.
Option 2: Fungus - Fungi are a separate kingdom of organisms, and they do not cause malaria. Fungi can cause diseases like fungal infections, but they are not responsible for malaria.
Option 3: Protozoan - Protists, specifically a group of protozoans called Plasmodium, are responsible for causing malaria. Plasmodium parasites are transmitted to humans through the bite of infected female Anopheles mosquitoes. Once inside the human body, these parasites invade the liver and red blood cells, leading to the symptoms of malaria.
Option 4: Virus - Viruses are also single-celled organisms, but they are not responsible for causing malaria. Viruses cause various other diseases, but not malaria.
In conclusion, option 3 is the correct answer as malaria is caused by the protozoan parasite Plasmodium.