Which of the following is not a membrane-bound organelle found in eukaryotic cells?

Ribosomes are not membrane-bound organelles. They are composed of RNA and protein and are responsible for synthesizing proteins within the cell. 

While ribosomes are essential for protein synthesis, they are not surrounded by a membrane and therefore are not considered to be a membrane-bound organelle.

In contrast, mitochondria, endoplasmic reticulum, and Golgi apparatus are all membrane-bound organelles found in eukaryotic cells.

Therefore, the correct answer is "ribosomes."