Net change in atomic number and mass number from sequential decays

Steps:

• α-emission decreases atomic number (Z) by 2 and mass number (A) by 4.
• First β-emission increases Z by 1, A unchanged.
• Second β-emission increases Z by 1, A unchanged.
• Net: Z unchanged (–2 +1 +1 = 0), A decreases by 4.

Why A is correct:

• Final nucleus has same Z (same element) but lower A (different isotope), per definition of isotopes as same Z, different A.

Why the others are wrong:

• B: Z is unchanged, so not higher proton number.
• C: Same element means same proton number (Z).
• D: A decreased by 4, so not identical.

Final answer: A