Which group consists of single-celled organisms without a nucleus?

The correct answer is that the group consisting of single-celled organisms without a nucleus includes both Archaea and Bacteria.

Both Archaea and Bacteria are categorized as prokaryotic organisms, which are defined by their lack of a membrane-bound nucleus. Instead of a nucleus, their genetic material is found in a nucleoid region within the cell. This distinction is fundamental in microbiology as it separates prokaryotes from eukaryotes, which do have a true nucleus.

Archaea are known for thriving in extreme environments, such as hot springs or high-salinity areas, and they exhibit distinct biochemical and genetic properties compared to bacteria. Bacteria are widespread and diverse, involved in various ecological roles, including nutrient cycling and potential pathogenicity.

Since both groups share the characteristic of being single-celled and lacking a nucleus, they are collectively grouped under the correct answer.