Correct order of field strength of given ligands is:

The correct answer is Option (3) → \(H_2O > OH^- > S^{2-}> SCN^- > I^-\)

Ligands split the d orbitals of the metal ion to different extents. Arranged in increasing order of field strength, they form the spectrochemical series, found experimentally from the light absorbed by their complexes. Placing the five ligands of option (3) in the series, from the strongest down:

• \( \ce{H2O} \) — strongest of the five; it comes after \( \ce{C2O4^{2-}} \) in the series.

• \( \ce{OH^{-}} \) — just below \( \ce{C2O4^{2-}} \), so weaker than \( \ce{H2O} \).

• \( \ce{S^{2-}} \) — below \( \ce{F^{-}} \) and \( \ce{OH^{-}} \).

• \( \ce{SCN^{-}} \) — below \( \ce{Cl^{-}} \) and \( \ce{S^{2-}} \).

• \( \ce{I^{-}} \) — the weakest ligand of the whole series.

NCERT (2026-27 edition):
Crystal Field Theory – “… In general, ligands can be arranged in a series in the order of increasing field strength as given below:
I^– < Br^– < SCN^– < Cl^– < S^2– < F^– < OH^– < C₂O₄^2– < H₂O < NCS^– < edta^4– < NH₃ < en < CN^– < CO
Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands.”