Isotopes share atomic number but differ in mass number

Steps:

• Identify atomic number (Z) and mass number (A) for each nucleus from the table.
• Compare Z values: same Z indicates same element.
• Check A values: different A confirms isotopes.
• Select pair with matching Z and differing A.

Why D is correct:

• R and S have the same atomic number (protons) but different mass numbers (protons + neutrons), defining isotopes per atomic structure definition.

Why the others are wrong:

• A: P and Q have different atomic numbers, so different elements.
• B: P and S have different atomic numbers, so different elements.
• C: Q and R have different atomic numbers, so different elements.

Final answer: D