Which structure is found in eukaryotic but not prokaryotic cells?

The nucleus is a defining feature of eukaryotic cells, serving as the control center that houses the cell's genetic material (DNA). Eukaryotic cells, which include plants, animals, fungi, and protists, possess a membrane-bound nucleus that separates the genetic material from the rest of the cell's contents. This compartmentalization allows for more complex regulation of gene expression and cell function.

In contrast, prokaryotic cells, which include bacteria and archaea, lack a true nucleus. Instead, their genetic material is located in a region called the nucleoid, which is not surrounded by a membrane. This fundamental difference in cell structure is one of the key characteristics that distinguish eukaryotes from prokaryotes.

Other structures mentioned in the options, such as the cell wall and ribosomes, can be found in both eukaryotic and prokaryotic cells, although they may differ in composition and function. Cytoplasm is a common component in all types of cells, as it encompasses the fluid and organelles within the cell membrane. Thus, the presence of a nucleus is uniquely characteristic of eukaryotic cells.