Net change in mass number from decay emissions Steps:

• Beta decay emits an electron; mass number (A) unchanged, atomic number (Z) increases by 1.
• Alpha decay emits helium-4 nucleus; A decreases by 4, Z decreases by 2.
• Two beta emissions: net A unchanged, Z +2.
• One alpha emission: A -4, Z -2.
• Net effect: A decreases by 4, Z unchanged (still Pb); standard case is Pb-212 to Pb-208 via thorium series.

Why B is correct:

• In thorium-232 decay chain, Pb-212 undergoes net two beta decays and one alpha decay to form Pb-208 (A = 212 - 4).

Why the others are wrong:

• A. 200: Requires net A decrease of 12 (e.g., three alphas), not matching emissions.
• C. 212: Implies no net A change, ignoring alpha decay.
• D. 216: Implies A increase of 4, impossible from these emissions.

Final answer: B