The correct sequence of ligands in the order of decreasing field strength is :

Solution

Chemistry ma’am · 3 steps

Fix the ends of the series first: CO is the strongest, halides and sulphide sit at the weak end.

1. Spectrochemical series. Ligands are ranked by how much they split the d orbitals, that is, by the size of Δₒ. Here is the order from weakest to strongest field.

   I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO

2. Why the ends. CO and CN⁻ accept electron density back from the metal through π bonding, so they give the largest splitting. Halides and S²⁻ donate extra π electrons, which shrinks the splitting, so they sit at the weak end.

   Option	Order given	Direction it really runs
   (a)	NCS⁻ > EDTA⁴⁻ > CN⁻ > CO	weakest to strongest, so reversed
   (b)	CO > H₂O > F⁻ > S²⁻	strongest to weakest, correct
   (c)	S²⁻ > OH⁻ > EDTA⁴⁻ > CO	weakest to strongest, so reversed
   (d)	OH⁻ > F⁻ > NH₃ > CN⁻	puts the strongest, CN⁻, last, so wrong

3. So. Only (b) runs from the strongest ligand down to the weakest: CO > H₂O > F⁻ > S²⁻.

The idea to keep. Remember three zones: halides and S²⁻ weak, H₂O and NH₃ middle, CN⁻ and CO strongest. Then check which way each option runs.

Answer from the NTA final answer key. Solution by PrepWiser.