What characteristic features define bacteria?

Bacteria are defined primarily by their cellular structure and organization. The correct choice highlights several key characteristics of bacteria: they are single-celled organisms, meaning each bacterium consists of just one cell, which differentiates them from multicellular organisms. Additionally, bacteria lack a nucleus, placing them in the category of prokaryotes. Prokaryotic cells do not have membrane-bound organelles, which is a fundamental distinction from eukaryotic cells. Furthermore, many bacteria possess a cell wall, which provides structural support and protection. This cell wall is crucial for maintaining shape and integrity, especially in varying environmental conditions.

The other options present characteristics that do not accurately reflect bacteria. For instance, bacteria are not multicellular (as per the first option) nor do they have a nucleus. While some bacteria may reside in vertebrates, they are ubiquitous and can be found in various environments, including soil, water, and extreme habitats, so the assertion that they are only found in vertebrates is misleading. Lastly, bacteria have diverse reproductive strategies, including both aerobic and anaerobic processes, making the claim of reproduction solely during anaerobic conditions inaccurate. Understanding these fundamental characteristics helps clarify the unique nature of bacterial life forms.