* Eukaryotic cells have a true nucleus, which is a membrane-bound organelle that houses the cell's DNA. This nucleus is a defining characteristic of eukaryotic cells.

* 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) are surrounded by a cell membrane: Both eukaryotic and prokaryotic cells have a cell membrane.

* c) genetic information: Both eukaryotic and prokaryotic cells contain genetic information (DNA). The difference lies in how this information is organized and contained within the cell.

In summary, the presence of a nucleus is the primary difference between eukaryotic and prokaryotic cells.