The energy source for cells is the:

Cells derive their energy primarily from mitochondria, often referred to as the "powerhouses of the cell." Mitochondria are organelles that generate adenosine triphosphate (ATP), the molecule that serves as a major energy currency in biological systems. This process occurs through cellular respiration, where nutrients, particularly glucose, are broken down in the presence of oxygen to produce ATP, carbon dioxide, and water.

The unique structure of mitochondria, including their double membrane and specialized enzymes, facilitates efficient energy production. Additionally, the presence of their own DNA and the ability to replicate independently further highlights their crucial role in energy metabolism.

While the Golgi apparatus, endoplasmic reticulum, and nucleolus are important for other cellular functions—such as protein processing, lipid synthesis, and ribosome production—none of them are directly responsible for energy generation in the same way mitochondria are. Therefore, it is the mitochondrion that correctly represents the primary energy source for cells.