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It would be S, which is Sulfur

The atom that would gain two electrons to fill its valence energy level is S(sulfur)

This is because s (sulfur) is in atomic number 16 with 2.8.6 of [Ne] 3s^2 2p^4 electronic configuration. This implies that sulfur has 6 valence electron and therefore it require two electron to fill its valence energy level and obtain 18 rule electrons.