Organisms whose cells contain a nucleus are called eukaryotes.

Here's why:

* Nucleus: The nucleus is a membrane-bound organelle that contains the cell's genetic material (DNA) in the form of chromosomes.

* Eukaryotic cells: These cells are complex and have membrane-bound organelles, including a nucleus.

* Prokaryotic cells: These cells are simpler and lack a nucleus. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

Examples of eukaryotic organisms include:

* Animals

* Plants

* Fungi

* Protists

Examples of prokaryotic organisms include:

* Bacteria

* Archaea