In which domain would you classify organisms with a complex cellular structure that includes a nucleus?

Organisms with a complex cellular structure that includes a nucleus are classified in the domain Eukarya. This domain encompasses all eukaryotic organisms, which are characterized by their cells containing a defined nucleus and membrane-bound organelles. This organizational complexity allows for more specialized cellular functions and greater versatility in processes such as metabolism, cellular respiration, and reproduction.

In contrast, the other domains represent organisms with simpler cellular structures. Bacteria and Archaea are prokaryotic, meaning their cells lack a nucleus and have a more simplistic organization. Viruses are not classified within these domains as they are not cellular organisms; they lack the necessary cellular machinery to sustain life independently and must infect host cells to replicate. Therefore, only Eukarya encompasses organisms with the defined complexity specified in the question.