Bacteria are best described as

• A. Large, multicellular organisms.
• B. Viruses.
• C. A type of fungus.
• D. Small, single-celled organisms that occur in almost any natural environment and can multiply to form groups or colonies.

Answer: (D)

Bacteria are small, single-celled organisms that are prokaryotes, meaning they lack a nucleus. They occur in almost every natural environment—from soil and water to inside living beings—and can multiply rapidly by binary fission, forming groups or colonies. This description best fits their basic nature: tiny autonomous cells capable of growth in a wide range of conditions. They are not large multicellular organisms, not viruses (which are acellular and need a host to replicate), and not fungi (which are eukaryotic).