Ans.
Mitochondria are membrane-bound organelles, present in eukaryotic cells. These are known as the power plant of the cells as they are site for biochemical reactions of respiration and produce energy currency (ATP) of the cell.
The reactions, involved in ATP production are Krebs cycle and oxidative phosphorylation. The Krebs cycle takes place in mitochondrial matrix, while oxidative phosphorylation occurs at inner mitochondrion membrane.
Thus, the correct answer is 'mitochondrion.'