The nuclear force is a short range force, this can be explained by fact that the binding energy per nucleon versus mass number curve
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → is flat for mass numbers in the range from 30 to 170.
The binding energy per nucleon is nearly constant, about 8 MeV, for nuclei of middle mass numbers (30 < A < 170). This flat part is the evidence for the short range of the nuclear force.
If every nucleon attracted every other nucleon in the nucleus, the binding energy per nucleon would keep growing as more nucleons were added. It does not, because a nucleon feels the attraction only of its few nearest neighbours within the range of the nuclear force; once a nucleus is larger than this range, adding nucleons does not change the binding of a given nucleon. This is called the saturation of the nuclear force.
The maximum near A = 56 and the fall for heavy nuclei are real features of the curve, but they describe nuclear stability and the Coulomb repulsion between protons; they do not show that the force is short-ranged.
NCERT (2026-27):
Nuclear Binding Energy – “… (ii) The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force is short-ranged. Consider a particular nucleon inside a sufficiently large nucleus. It will be under the influence of only some of its neighbours, which come within the range of the nuclear force.”