Step 1: Understanding Spectrochemical Series

The Spectrochemical Series is an experimentally determined arrangement of ligands in the order of their increasing field strength (or increasing crystal field splitting energy, $\Delta$). Ligands that cause larger splitting are called strong field ligands, while those causing smaller splitting are weak field ligands.

Step 2: Analyzing the Ligands

Based on the spectrochemical series, the relative field strengths are as follows: $$CO > CN^{-} > en > NH_{3} > H_{2}O > F^{-} > Cl^{-} > Br^{-} > I^{-}$$

Step 3: Comparing the Options

We need to arrange the given ligands ($Cl^{-}, NH_{3}, H_{2}O, CO$) in decreasing order of field strength:

• $CO$ is a strong $\pi$-acceptor ligand (strongest field).
• $NH_{3}$ is a strong $\sigma$-donor ligand.
• $H_{2}O$ is a weak field ligand.
• $Cl^{-}$ is a weak field halide ligand (weakest field).