What defines eukaryotic cells?

Eukaryotic cells are defined by having a nucleus that contains their genetic material, which is organized into multiple chromosomes. This distinguishes them from prokaryotic cells, which do not have a nucleus and usually possess a single circular chromosome. The presence of a well-defined nucleus allows for more complex regulation of gene expression and cell function. Additionally, eukaryotic cells can be unicellular or multicellular, exhibiting a wide range of complexity including the formation of tissues and organs in multicellular organisms.

The option stating the absence of a cell membrane is incorrect because all cells, including eukaryotes, have a cell membrane that encases the cell's contents. Additionally, eukaryotic cells are not limited to being unicellular; they can be part of larger organisms, which supports the diversity of life forms seen in both microscopic and macroscopic entities. Thus, the defining feature of eukaryotic cells is indeed their nucleus and the structure of their chromosomes.