The correct answer is: A. Plasmids
Plasmids are small, circular pieces of DNA that are found in bacteria. They are not essential for the survival of the bacterium, but they can confer certain advantages, such as resistance to antibiotics. Plasmids are usually replicated independently of the bacterial chromosome.
Genes are the basic units of heredity. They are located on chromosomes, which are long, linear molecules of DNA. Genes code for proteins, which are the building blocks of cells and tissues.
Chromosomes are found in the nucleus of eukaryotic cells. They are made up of DNA and proteins. Chromosomes are responsible for storing and transmitting genetic information.
Mesosomes are invaginations of the plasma membrane that are found in bacteria. They are involved in cell division, respiration, and transport.
Plasmids are the only option that is a piece of DNA. Genes, chromosomes, and mesosomes are not pieces of DNA.