The nuclei have maximum binding energy per nucleon whose mass number lies between:

Explanation

The nuclei have maximum binding energy per nucleon whose mass number lies between 50 and 60. This is because the nuclear force, which is the force that binds protons and neutrons together in a nucleus, is a short-range force. For nuclei with mass number in this range, the distance between nucleons is not too large, so the nuclear force is strong enough to bind them together. However, for nuclei with mass number outside this range, the distance between nucleons becomes too large, and the nuclear force is not strong enough to bind them together as tightly. As a result, the binding energy per nucleon decreases for nuclei with mass number outside this range.