Nucleons total protons plus neutrons, equaling the mass number Steps:

• Krypton's atomic number is 36, so it has 36 protons in a neutral atom.
• The isotope "Kr" refers to the common Kr-84, with mass number 84.
• Neutrons = mass number - atomic number = 84 - 36 = 48.
• Total nucleons = protons + neutrons = 36 + 48 = 84.

Why C is correct:

• By definition, the mass number of an isotope is the total number of nucleons (protons + neutrons).

Why the others are wrong:

• A: Equals only the number of protons (atomic number), ignoring neutrons.
• B: Matches neutrons in Kr-84 but not total nucleons.
• D: Exceeds the mass number of any stable krypton isotope.

Final answer: C