Final answer:
The finite range of the strong nuclear force means that it only binds nucleons effectively within a certain distance, reaching optimal binding energy per nucleon (BE/A) for nuclei with mass numbers near 60, like iron.
Step-by-step explanation:
The finite range of the nuclear force is crucial in understanding why the binding energy per nucleon (BE/A) is greatest for atomic mass numbers (A) close to 60. The strong nuclear force, which is about 100 times stronger than the Coulomb force, is a short-range force that allows nucleons (protons and neutrons) to be bound within the nucleus. For nuclei with low-mass numbers, every added nucleon can interact with all the other nucleons via the strong nuclear force, which leads to an increase in BE/A.
However, as A approaches 60, which is roughly the mass number of iron, the finite range of the strong nuclear force limits the stability because nucleons added beyond this point will be too distant to feel the nuclear attraction strongly. This limit results in the stabilization of intermediate-mass nuclei, as they are compact enough for all nucleons to be within the range of the strong nuclear force. Therefore, the correct answer to the question is that (a) a short-range force limits stability; BE/A peaks for intermediate nuclei.