What are autonomously replicating small circles of DNA found in some bacteria called?

Plasmids are indeed autonomously replicating small circles of DNA that are commonly found in bacteria. They are separate from the chromosomal DNA, which is typically larger and linear in structure. Plasmids serve various functions, often carrying genes that provide beneficial traits to the bacteria, such as antibiotic resistance or the ability to metabolize specific nutrients. Because of their capacity to replicate independently of the chromosomal DNA, they can be easily transferred between bacteria, contributing to genetic diversity and adaptability in microbial populations.

In the context of the other options, chromosomes refer to the larger, linear structures within the cell that contain the main genetic material, making them distinct from plasmids. Viruses are entities that require a host to replicate and do not have the capability to independently replicate within a bacterial cell as plasmids do. Genomes encompass the complete set of genetic material in an organism, including both chromosomal and plasmid DNA in the case of bacteria, but they are not specifically the small circles of DNA themselves. Therefore, the term that accurately describes these small circular DNA molecules within some bacteria is plasmids.