Answer
The energy associated with the strong force between nucleons in a nucleus is proportional to A.
Work Step by Step
Since nucleons of strong force in a nucleus interact with nearby nucleons at short range of $1.0 \times 10^{-15}m $ , it cannot interact with nucleons that are far from it.
Let's assume N is the number of neighbors that interact with any nucleon. N is independent to the mass number (A) in the nucleus.
The number of interactions in a nucleus is approximately $NA$, so the energy associated with the strong nuclear force is proportional to $NA$, hence proportional to $A$ itself.
