Net changes in atomic and mass numbers from decay sequence

Steps:

• β⁻ decay (W to X): atomic number increases by 1 (neutron → proton), mass number unchanged.
• α decay (X to Y): atomic number decreases by 2, mass number decreases by 4 (loss of ²⁴He nucleus).
• β⁺ decay (Y to Z): atomic number decreases by 1 (proton → neutron + positron), mass number unchanged.
• Net effect: atomic number change = +1 – 2 – 1 = –2 (different element); mass number change = 0 – 4 + 0 = –4 (lower nucleon number).

Why C is correct:

• Z has atomic number 2 less than W (different element by definition) and nucleon number (protons + neutrons = mass number) 4 less, per standard decay rules.

Why the others are wrong:

• A: Nucleon number decreases by 4, not increases.
• B: Atomic number differs by 2, so not same element.
• D: Element differs and nucleon number is lower, not higher.

Final answer: C