Answer:
a) the covalent bond between the last phosphate and the middle phosphate
Explanation:
The ADP is composed by a molecule of Adenine, Ribose and two Phosphate groups. The addition of a paper weight in the spring represents the last phosphate added in a ADP, resulting in a ATP. The last phosphate connected with Ribose by a covalent bond, represented by the round paper weight connected to a spring, can be easily detached in order to provide energy to the cell by a process called dephosphorylation. The spring compressed represents the energy of the bond. In the model the compressed spring is represents the potential energy, but in the bond the energy is chemical, which will lead into a electron transference reaction.