Composition Of Nucleus

As NCERT explains, a nucleus is described by its mass number $A$, the total count of protons and neutrons, and its atomic number $Z$, the number of protons which also equals the number of electrons in the neutral atom. The neutron number is therefore $N=A-Z$. For iron-56, $A=56$ and $Z=26$, so $N=56-26=30$ neutrons. The value 26 is wrong because that is the proton (atomic) number, not the neutron count. The value 56 is wrong because that is the total nucleon number, which includes the protons as well. The value 82 is wrong because it adds $A$ and $Z$ instead of subtracting, which has no physical meaning here. As stated in NCERT Class 12, Chapter 13 (Nuclei), isotopes of an element share the same $Z$ but differ in $N$ and hence in $A$. A plausibility check: iron-56 is one of the most tightly bound nuclei, and its near-equal proton and neutron counts (26 and 30) are typical of stable medium-mass nuclei.