Neutrons in nuclides = mass number - atomic number

Steps:

• Identify atomic numbers: radon (Rn) = 86, francium (Fr) = 87, radium (Ra) = 88.
• For Rn-211: neutrons = 211 - 86 = 125.
• For Fr-210: neutrons = 210 - 87 = 123.
• For Ra-212: neutrons = 212 - 88 = 124.

Why B is correct:

• B gives 125, 123, 124, matching the formula N = A - Z for each nuclide.

Why the others are wrong:

• A: Uses incorrect differences (e.g., 211 - 83 = 128, mistaking Rn's Z).
• C: Lists mass numbers A, which include protons and neutrons.
• D: Yields impossible values exceeding reasonable nuclear sizes.

Final answer: B