The correct answer to the question is option 4: Virus. Dengue fever is caused by the dengue virus, which belongs to the Flaviviridae family. It is transmitted to humans through the bite of infected Aedes mosquitoes, primarily the Aedes aegypti species. Dengue viruses are single-stranded RNA viruses, and there are four distinct serotypes (DENV-1, DENV-2, DENV-3, and DENV-4).

Options 1, 2, and 3 are incorrect. Fungus, bacteria, and protozoa are not responsible for causing dengue fever. It is important to note that dengue is a viral illness, not a microbial infection caused by these other types of organisms. Additionally, none of these organisms are transmitted to humans through mosquito bites, which is a characteristic feature of dengue transmission. Hence, option 4 remains the correct answer.