Final answer:
Viruses are the entities that may utilize RNA as their genome. They are distinct from other living organisms in having either DNA or RNA as genetic material, not both, with specific groups, like Group VI viruses, using RNA genomes that are reverse-transcribed into DNA for replication.
Step-by-step explanation:
The organism that may use RNA as its genome is c) Viruses. Unlike nearly all living organisms which use DNA for their genetic material, viruses have a diverse nature in terms of genetic composition. Viral genomes can either be constructed of DNA or RNA. In particular, certain types of viruses, such as Group I and Group VI, have different mechanisms of replication. Group I viruses contain DNA as their genome and produce RNA from this DNA template. On the other hand, Group VI viruses have RNA as their genome and utilize the enzyme reverse transcriptase to synthesize DNA from their RNA genome before they can replicate.
Viruses are extremely small, acellular entities that are only seen with an electron microscope, and they lack the cellular structure of living organisms. They replicate by using host cell machinery or proteins encoded in their own viral genome. Since viruses do not use both DNA and RNA simultaneously, they are unique in their genetic makeup. RNA replication in some viruses occurs with the aid of an enzyme called RNA-directed-RNA-polymerase.