The correct answer is that they produce ATP.
Mitochondria are considered an essential part of the cells as they consume food and produce energy in the process, which is utilized by the rest of the cells.
The prime activity of mitochondria is to generate energy for the cell. The cells utilize a specific molecule for energy known as ATP. The ATP is produced inside the mitochondria, that is, why one can consider mitochondria as the energy house of the cell.