Answer: A. Count the electrons in each ion:
\({}^{32}_{16}\text{S}^{2-}\): 16 protons, gained 2 electrons → 18 electrons.
\({}^{35}_{17}\text{Cl}^{-}\): 17 protons, gained 1 electron → 18 electrons.
\({}^{40}_{20}\text{Ca}^{2+}\): 20 protons, lost 2 electrons → 18 electrons.
All three ions are isoelectronic with argon (18 electrons).
B fails: neutrons are 16, 18, 20 — all different. C fails: protons are 16, 17, 20 — all different. D fails: S and Cl are in Period 3, but Ca is in Period 4.
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