Identification of Chemical Reagent S

Based on the comprehensive scheme of qualitative analysis from this passage (JEE Advanced 2014):

1. Analysis of the Passage Scheme: The paragraph presents a flow-chart involving reactions of metal cations with various specific qualitative inorganic reagents, designated as $\mathbf{P}$, $\mathbf{Q}$, $\mathbf{R}$, and $\mathbf{S}$.

2. Reactions and Identification:

   • Metal ions such as $\mathrm{Pb}^{2+}$, $\mathrm{Zn}^{2+}$, or amphoteric cations precipitate as hydroxides when treated with a strong base and dissolve in an excess of the reagent to form soluble tetrahydroxo-complexes (e.g., tetrahydroxozincate(II) or tetrahydroxoplumbate(II)): $\mathrm{M}^{2+} + 2\mathrm{OH}^- \longrightarrow \mathrm{M(OH)}_2 \downarrow$ $\mathrm{M(OH)}_2 + 2\mathrm{OH}^- \longrightarrow [\mathrm{M(OH)}_4]^{2-}$
   • This amphoteric behavior—forming a white precipitate that dissolves in an excess of the reagent to yield a clear, colorless solution—unambiguously identifies reagent $\mathbf{S}$ as a strong base, potassium hydroxide ($\mathrm{KOH}$).

3. Conclusion: Reagent $\mathbf{S}$ is $\mathrm{KOH}$.

Therefore, the correct option is (D).