Which organism type lacks a nucleus and membrane-bound organelles?

The correct answer is prokaryotes, which are single-celled organisms that lack a nucleus and membrane-bound organelles. In prokaryotes, such as bacteria, the genetic material is not enclosed in a nucleus but is present as a single, circular strand of DNA located in a region called the nucleoid. Additionally, they do not possess the complex structures found in eukaryotic cells, such as mitochondria, endoplasmic reticulum, or Golgi apparatus, which are all membrane-bound organelles crucial for various cellular functions in eukaryotes.

In contrast, eukaryotes are characterized by the presence of a nucleus that contains their genetic material, as well as various membrane-bound organelles. Viruses, while they may exhibit some characteristics of living organisms, are not classified as cells and do not possess cellular structures like organelles or nuclei; instead, they consist of genetic material surrounded by a protein coat. Fungi are also eukaryotic organisms, possessing a defined nucleus and other organelles. Thus, prokaryotes are distinct in their simpler cellular structure, highlighting the absence of a nucleus and membrane-bound structures.