Ar (18) = 1s²2s²2p⁶3s²3p⁶. Cl⁻ (18) = 1s²2s²2p⁶3s²3p⁶. The question asks for 3s²3p⁴ which corresponds to Sulfur (S) or similar, but based on the provided options, Cl⁻ is the closest match for 18 electrons, though the question text in the image shows 3s²3p⁴ which is S. Given the options, there might be a typo in the source question, but Cl⁻ is the intended answer in the provided context.