Nucleons equal the mass number of an isotope

Steps:

• The superscript in ^{84}Kr gives the mass number A = 84, defined as total protons + neutrons.
• Krypton's atomic number Z = 36, so protons = 36.
• Neutrons = A - Z = 84 - 36 = 48.
• Total nucleons = protons + neutrons = 84.

Why C is correct:

• By definition, the mass number A directly equals the total number of nucleons in the nucleus.

Why the others are wrong:

• A: 36 is the atomic number Z, counting only protons.
• B: 48 is the number of neutrons (A - Z), not total nucleons.
• D: 120 is Z + A, an irrelevant sum with no physical meaning here.

Final answer: C