Which characteristic defines bacteria?

Bacteria are characterized by their prokaryotic nature, which means they lack a true nucleus. Instead of being enclosed within a nuclear membrane, bacterial DNA is organized in a single circular chromosome that resides in the cytoplasm of the cell. This absence of a nucleus is a fundamental distinction between prokaryotic organisms, such as bacteria, and eukaryotic organisms, which have a defined nucleus and numerous membrane-bound organelles.

The other characteristics outlined in the options are not defining traits of bacteria. For instance, multicellular organization is typical of eukaryotic organisms, and most bacteria are single-celled (unicellular) organisms. The presence of membrane-bound organelles, such as mitochondria or endoplasmic reticulum, is also exclusive to eukaryotic cells, not bacteria. Finally, while some bacteria can exchange genetic material through processes like conjugation, they primarily reproduce asexually via binary fission, making the concept of sexual reproduction irrelevant in the context of defining bacteria. Thus, the defining characteristic that sets bacteria apart is their lack of a nucleus.