The cell organelle responsible for producing energy in eukaryotic cells is the mitochondrion.

Here's why:

* Cellular Respiration: Mitochondria are the sites of cellular respiration, the process that breaks down glucose (sugar) in the presence of oxygen to produce ATP (adenosine triphosphate). ATP is the primary energy currency of cells.

* Powerhouse of the Cell: Mitochondria are often called the "powerhouses of the cell" because they generate most of the cell's ATP.

While other organelles contribute to cellular processes that indirectly involve energy, the mitochondrion is the primary energy-producing organelle in eukaryotic cells.