Which of the following is an RNA virus?

Orthomyxoviridae is indeed classified as an RNA virus. This family includes viruses that are responsible for influenza in humans and animals. The hallmark of RNA viruses like those in the Orthomyxoviridae family is that they contain ribonucleic acid as their genetic material rather than deoxyribonucleic acid (DNA).

In contrast, the other families listed—Herpesviridae, Polyomaviridae, and Poxviridae—are characterized by their DNA genomes. Herpesviridae includes viruses such as herpes simplex virus and varicella-zoster virus, while Polyomaviridae includes polyomaviruses, which are also DNA-based. Poxviridae, which includes variola virus (the causative agent of smallpox), also contains DNA. Therefore, the correct identification of Orthomyxoviridae as an RNA virus is based on its unique genetic structure, distinguishing it from the DNA viruses in the other families mentioned.