What kingdom of organisms is characterized by the absence of a membrane-bound nucleus?

The kingdom that is characterized by the absence of a membrane-bound nucleus is Monera. Organisms in this kingdom are prokaryotic, meaning they lack a true nucleus and other membrane-bound organelles. Instead, their genetic material is typically organized in a single circular strand of DNA that resides in the cytoplasm. This fundamental characteristic distinguishes prokaryotes from eukaryotes, which comprise the other kingdoms mentioned.

In contrast, the kingdoms Protista, Animalia, and Fungi consist of eukaryotic organisms that do possess a membrane-bound nucleus and various organelles, which is a key feature of cells in these kingdoms. Thus, Monera stands out as the only kingdom where organisms are defined by their prokaryotic structure.