JEE Main · 2024 · 04 Apr 2024 · Shift 2 · Chemical Bonding and Molecular Structure

Considering the geometry around the central sulfur atom in each species:

• $S{O}_3^{2-}$ (sulfite): S has 3 bonded O atoms plus one lone pair — pyramidal (AX${}_3$E), like $N{H}_3$.
• $S_2{O}_3^{2-}$ (thiosulfate), $S{O}_4^{2-}$ (sulfate) and $S_2{O}_7^{2-}$ (pyrosulfate): in each of these the central S atom is bonded to four groups (four O atoms, or three O atoms plus the terminal S in thiosulfate) with no lone pair — tetrahedral (AX${}_4$) about that centre.

Only $S{O}_3^{2-}$ is pyramidal, so the count is 1.
(This question was flagged for possible ambiguity depending on whether "geometry" is read as molecular shape or electron-pair geometry; the NTA/Allen key treats only the sulfite ion as pyramidal, giving 1.)