Among the ligands `NH_3`, `en`, `CN^(-)` and `CO`, the correct order of their increasing field strength, is

Options

A. (a) `COltNH_3ltenltCN^(-)`
B. (b) `NH_3ltenltCN^(-)ltCO`
C. (c) `CN^(-)ltNH_3ltCOlten`
D. (d) `enltCN^(-)ltNH_3ltCO`

Explanation

To determine the correct order of increasing ligand field strength among the ligands \(NH_3\), \(en\) (ethylenediamine), \(CN^-\), and \(CO\), we can refer to the spectrochemical series, which ranks ligands based on their ability to split the d-orbitals in transition metal complexes.

### Step-by-Step Solution:

1. **Understanding Ligand Field Strength**:
- Ligand field strength refers to the ability of a ligand to cause splitting of the d-orbitals in a transition metal complex. Strong field ligands cause a larger splitting of the d-orbitals compared to weak field ligands.

2. **Referencing the Spectrochemical Series**:
- The spectrochemical series is a list that ranks ligands based on their field strength. The order from weakest to strongest field ligands is as follows:
- \(I^- < Br^- < SCN^- < Cl^- < S^{2-} < F^- < OH^- < C_2O_4^{2-} < H_2O < EDTA^{4-} < NH_3 < en < NO_2^- < CN^- < CO\)

3. **Identifying the Position of Each Ligand**:
- From the spectrochemical series, we can identify the positions of the given ligands:
- \(NH_3\) is positioned before \(en\), indicating that \(NH_3\) has a lower field strength than \(en\).
- \(en\) is positioned before \(CN^-\), indicating that \(en\) has a lower field strength than \(CN^-\).
- \(CN^-\) is positioned before \(CO\), indicating that \(CN^-\) has a lower field strength than \(CO\).

4. **Arranging the Ligands in Order of Increasing Field Strength**:
- Based on the positions identified:
- The order of increasing field strength is:
\[ NH_3 < en < CN^- < CO \]

### Final Answer:
The correct order of increasing field strength among the ligands \(NH_3\), \(en\), \(CN^-\), and \(CO\) is:
\[ NH_3 < en < CN^- < CO \]