Isotopes share protons but differ in neutrons

Steps:

• Isotopes are atoms of the same element, defined by identical atomic numbers.
• Atomic number equals the number of protons, so P and Q have the same protons.
• The "35" likely indicates mass number for P, implying Q has a different mass number.
• Mass number is protons plus neutrons, so different mass numbers mean different neutrons.

Why C is correct:

• By definition, isotopes have the same number of protons (same element) but different numbers of neutrons (different mass numbers).

Why the others are wrong:

• A: Different protons would mean different elements, not isotopes.
• B: Different protons contradict the same-element definition.
• D: Same protons and neutrons would make them identical atoms, not isotopes.

Final answer: C