Option 1: Insects are not the direct cause of malaria. However, they play a significant role in its transmission. Female Anopheles mosquitoes are the primary vector that carries and transmits the malaria-causing parasites when they bite infected humans.

Option 2: Bacteria, such as the ones that cause diseases like tuberculosis or strep throat, do not cause malaria. Malaria is caused by a different type of microorganism.

Option 3: Protozoa is the correct answer. Malaria is caused by a group of single-celled parasites called Plasmodium. When infected female Anopheles mosquitoes bite humans, they inject these parasites into the bloodstream, leading to malaria.

Option 4: Viruses are not responsible for causing malaria. Viral infections are caused by a different group of microorganisms and have different symptoms and treatments.

Therefore, the correct answer is option 3 - Protozoa.