* Eukaryotic cells have a true nucleus, which houses their DNA and controls cellular activities.

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

Let's look at the other options:

* A. Mitochondria: While mitochondria are the powerhouses of eukaryotic cells, they are also found in some prokaryotic cells.

* B. Golgi apparatus: The Golgi apparatus is involved in processing and packaging proteins, and it is found in eukaryotic cells but not in prokaryotes.

* D. Cell membrane: Both eukaryotic and prokaryotic cells have a cell membrane that regulates what enters and exits the cell.

In summary: The presence of a nucleus is the defining characteristic that differentiates eukaryotic cells from prokaryotic cells.