The kingdom that contains organisms with only one cell (unicellular) and no nucleus is called Monera.

Here's why:

* Monera is a kingdom that includes bacteria and archaea.

* Bacteria and archaea are prokaryotes.

* Prokaryotes are single-celled organisms that lack a true nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, but it's not enclosed by a membrane.

Let me know if you have other questions about kingdoms or organisms!