Which part of the molecule holds the energy waiting to be released

Energy awaiting to be released is held in the bond between the 2nd and the 3rd phosphate groups of ATP.

Explanation:

The ATP molecule has an adenosine molecule that is made up of three phosphate groups.

Energy produced during respiratiojn is stored in the bond between the second ans the third phosphate groups, when needed to be used by the cell, ATP looses its third bond exposing the stored energy for use.

Through the above process, it goes back to the ADP form that combines with a third phosphate group to store energy.