(A) : The nuclear force between two nucleons falls rapidly to zero as their distance is more than a few femtometer.
(R) : Sharp decrease in nuclear forces made it saturated, which causes constancy for binding energy per nucleon.

As soon as two nucleons are separated by more than a few femtometers, the nuclear force between them quickly vanishes. The constant binding energy per nucleon results from the saturation of forces in a medium or large-sized nucleus as a result of this.

Both (A) and (R) are true and (R) is the correct explanation of (A).