Bacteria are

• A. Large, multicellular organisms.
• B. Viruses that infect cells.
• C. A type of mold.
• 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 occur in almost any natural environment and can multiply to form groups or colonies. They are prokaryotes, meaning their cells lack a nucleus, and they often reproduce quickly, sometimes making visible colonies when grown in suitable conditions. This description fits the idea that bacteria are tiny, individual cells that can come together to form larger groups. The other statements don’t fit because large, multicellular organisms describe plants or animals; viruses are not cells at all and require a host to reproduce; and molds are fungi, which are eukaryotic and typically multicellular.