KEAM 2010 Physics and Chemistry Paper

(i) Isotopes of an element are nuclides having same atomic number Z but different mass number A.
(ii) Nuclides having same mass number A but different atomic number Z are called isobars.
(iii) Nuclides with different atomic number Z and different mass number A but same neutron number are called isotones. Thus, for isotones N=(A−Z) is constant. Hence, option (d) is correct.