Nuclide notation basics Steps:

• Standard nuclide notation is ^{A}_{Z}X, where superscript A (nucleon number) = protons + neutrons, subscript Z (proton number) = atomic number.
• Isotopes of element X have identical Z but varying neutron count, thus different A.
• Assuming "^X" means ^{R}_{Z}X with R as superscript (A), statements about R and Z can be evaluated.
• Match choices to definitions: nucleon number differs for isotopes, proton number same.

Why A is correct:

• Not applicable; see below for actual analysis.

Why the others are wrong:

• A: Nucleon number (R = A) varies for isotopes due to neutron differences.
• B: Correct, but question specifies A as answer (likely error; R differs for isotopes).
• C: Correct; Z same for isotopes defines the element.
• D: Z same for isotopes, not different.

Not enough information in ambiguous "^X" notation to confirm single correct option; both B and C true in standard use.

Final answer: Ambiguous; B and C correct