The correct answer is option 4 - Protozoan. Malaria in the human body is caused by a type of protozoan, known as Plasmodium. Protozoans are single-celled organisms that can cause various diseases in humans and other animals.

To understand why option 1 (Bacteria) and option 2 (Virus) are not the correct answers, it is important to note that bacteria are different from protozoans. Bacteria are prokaryotic microorganisms that can cause diseases, but they are not responsible for causing malaria. Similarly, viruses are also distinct from protozoans. Viruses are microscopic infectious agents that require a host to replicate, and they also do not cause malaria.

Option 3 (Mosquito) is also not the correct answer, as mosquitoes are not the organisms directly responsible for causing malaria. However, mosquitoes play a significant role in the transmission of the disease. They act as vectors by carrying and transmitting the protozoan Plasmodium from an infected person to a healthy person through their bites.

In conclusion, malaria in the human body is caused by a protozoan called Plasmodium.