Explanation

Dengue fever is caused by the dengue virus, a mosquito-borne virus transmitted primarily by Aedes aegypti. It belongs to the genus Flavivirus in the family Flaviviridae, and is a positive-sense single-stranded RNA virus — hence the "Flavi-ribo virus" category. Adenoviruses cause respiratory and eye infections, vaccinia virus is related to smallpox, and Nipah virus causes encephalitis; none of these cause dengue. The answer is (A).