Answer
Only choice A is true. As the phosphate group approaches the ADP molecule, it is first repelled, and then attracted.
Work Step by Step
Starting at large r, we see that as the phosphate group approaches the ADP molecule, PE rises. This results in a repulsive force. Eventually, the PE decreases as the phosphate group approaches still closer, and this is an attractive force. Thus we see that “A” is true; as the phosphate group approaches the ADP molecule, it is first repelled and then attracted.
B and C are false because the force is not always attractive, nor always repulsive.
D is false. Starting at large r, we see that as the phosphate group approaches the ADP molecule, PE first rises, then falls.
The phosphate group must have had some initial KE to overcome the repulsive force. The system first loses KE, gaining PE. Then the PE falls as the system settles into its bound state. Overall, the system has more PE than it does when the phosphate group and ADP are separated.
In summary, in forming the bond, the system first gains PE, then loses PE (it loses a smaller amount than what was initially stored). This is the opposite of what D says, so D is false.
E is false because the ADP molecule has a positive activation energy, i.e., the phosphate group must have had some initial KE to overcome the repulsive force.
F is false because the binding energy is negative. The energy of the bonded molecule is larger than when the phosphate group and ADP are separated.
