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

🧠 The Three-Tier Mental Map

Spectrochemical series problems sound like memory games, but they collapse to a single picture: a three-tier ladder. Bottom rung — heavy/diffuse donors ($\mathrm{I^-}$, $\mathrm{S^{2-}}$, halides). Middle rung — hard O/N donors ($\mathrm{H_2O}$, $\mathrm{OH^-}$, $\mathrm{NH_3}$, en). Top rung — $\pi$-acceptors ($\mathrm{CN^-}$, $\mathrm{CO}$). Once the rungs are in your head, "decreasing order" is just reading top-to-bottom.

🗺️ Walk the Ladder

Take each option and check whether it walks down the ladder.

(1) $\mathrm{NCS^- > EDTA^{4-} > CN^- > CO}$ — places $\mathrm{CO}$ last. $\mathrm{CO}$ is the strongest field ligand we routinely meet. Wrong direction.

(2) $\mathrm{CO > H_2O > F^- > S^{2-}}$ — top rung → middle rung → halide → sulfide. Clean descent across all three tiers.

(3) Starts with $\mathrm{S^{2-}}$ as strongest. $\mathrm{S^{2-}}$ is among the weakest — disqualified instantly.

(4) Places $\mathrm{CN^-}$ last. $\mathrm{CN^-}$ is near the top. Wrong.

⚡ The 5-Second Filter

Whenever the option ranks $\mathrm{CO}$ or $\mathrm{CN^-}$ as weakest, or $\mathrm{S^{2-}}/\mathrm{I^-}$ as strongest, kill it without further thought. In four-option MCQs this alone usually leaves one survivor.

⚠️ The Trap That Catches Most

Students confuse "field strength" with "basicity / charge". $\mathrm{S^{2-}}$ has a $-2$ charge, $\mathrm{CO}$ is neutral — yet $\mathrm{CO}$ wins by a mile. The reason is $\pi$-back-donation: empty $\pi^*$ orbitals on $\mathrm{CO}$ accept metal $\mathrm{d}$-electron density, dramatically widening $\Delta$. Charge alone does not predict the series.

$\boxed{\text{Answer: (2) } \mathrm{CO > H_2O > F^- > S^{2-}}}$