Which of the following statements best defines bacteria?

The statement that best defines bacteria is that they are single-cell microorganisms that reproduce by cell division. Bacteria are classified as prokaryotic organisms, meaning they do not have a nucleus or other membrane-bound organelles. This fundamental characteristic sets them apart from eukaryotic cells, which are found in multicellular organisms, including plants and animals. The ability of bacteria to reproduce by cell division, specifically through a process called binary fission, allows them to multiply rapidly under favorable conditions, leading to significant populations in various environments, including soils, water, and even within the human body.

The other options provide definitions or characteristics that are not accurate or do not encompass the broader understanding of bacteria. While some bacteria may exist in extreme environments, many thrive in regular conditions. Additionally, not all bacteria lack chlorophyll, nor are they primarily defined by the ability to photosynthesize, which is a trait more commonly associated with specific types of bacteria such as cyanobacteria. Thus, the definition focusing on their single-celled nature and mode of reproduction provides the most accurate and comprehensive understanding of bacteria.