Since the options are given I'd describe ATP.
ATP usually releases energy by breaking down the weak phosphate cell present. The energy released due to the breaking down of the phosphate cell is then utilized. The ATP is converted to ADP. Whenever a cell feels the necessity for energy it releases a phosphate to gather the energy. It is an automatic and ongoing process. The weak bond between the second and the third phosphate is always targeted for turning ATP to ADP. This is the reason ATP is considered the main energy currency for the cell.
