Which type of cellular organisms lack a true nucleus?

The correct answer is that prokaryotes are the type of cellular organisms that lack a true nucleus. Prokaryotic cells are characterized by the absence of membrane-bound organelles, including a defined nucleus. Instead of a nucleus, prokaryotes have a nucleoid region where their genetic material (DNA) is located. This simpler structure distinguishes them from eukaryotic cells, which do have a true nucleus enclosed by a nuclear membrane.

In contrast, eukaryotes, which include organisms such as protists and fungi, possess a well-defined nucleus that houses their genetic material. Protists, being a subset of eukaryotes, also have this characteristic. Fungi, similarly, are eukaryotic and feature a true nucleus. Thus, prokaryotes are unique in lacking this structural complexity, making them distinct in the classification of cellular organisms.