C. a small, circular piece of DNA
A plasmid is a small, circular piece of DNA found in bacteria and other single-celled organisms. It is separate from the organism's main chromosomal DNA and can exist in multiple copies per cell. Plasmids are often used as a vector for genetic engineering. They can be easily isolated from a bacterial cell and manipulated in a laboratory. Genetic engineers use plasmids to introduce new genetic material into cells, such as genes for resistance to antibiotics or for the production of a desired protein. Plasmids can also be used to transfer genetic information between different species of organisms.