According to crystal field theory, the correct order of ligands with respect to their decreasing field strength is:

Solution & Explanation

Concept: ligand field strength is ranked by the SPECTROCHEMICAL SERIES, which orders ligands by how large a crystal-field splitting (Δ) they produce. The relevant part of the series (weak field → strong field) is: Cl⁻ < F⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO. Key points: halides like Cl⁻ are weak-field; H₂O is moderate; NH₃ is stronger than H₂O (it is a better σ-donor and not a π-donor); CO is the strongest here because it is a powerful π-acceptor. Writing the four given ligands in DECREASING field strength: CO > NH₃ > H₂O > Cl⁻. This matches answer (A). Trap: a common error is putting H₂O above NH₃ — remember NH₃ lies higher (stronger field) than H₂O in the series.