Isotopes share atomic number but differ in mass number

Steps:

• Recall isotopes are atoms of the same element with identical proton count (atomic number) but different neutron count (mass number).
• Examine particle data for atomic and mass numbers (assuming standard labeling; data not provided here).
• Compare proton numbers across pairs to find matches.
• Identify the pair with same protons but different total nucleons.

Why D is correct:

• Y and Z have the same atomic number per isotope definition, confirming they are variants of the same element.

Why the others are wrong:

• A: W and X have different atomic numbers, so different elements.
• B: W and Z have different atomic numbers, so different elements.
• C: X and Z have different atomic numbers, so different elements.

Not enough information: Particle specifics (e.g., proton/neutron counts) missing from query.

Final answer: D