The decreasing nucleophilicity of the following is :
(I) `CH_3 S^(Ө)`
(II) `CH_3 O^(Ө)`
(III) `overset (Ө) O H`
(IV) `EtO^(Ө)`.

Options

A. `IV gt III gt II gt I`
B. `I gt II gt III gt IV`
C. `IV gt III gt I gt II`
D. `II gt I gt III gt IV`

Explanation

To determine the decreasing nucleophilicity of the given species, we need to analyze their corresponding acids and the relationship between acidity and basicity, which in turn affects nucleophilicity. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Species and Their Corresponding Acids
The given nucleophiles are:
1. \( CH_3S^- \) (methyl thiolate)
2. \( CH_3O^- \) (methoxide)
3. \( OH^- \) (hydroxide)
4. \( EtO^- \) (ethoxide)

We can identify their corresponding acids:
- \( CH_3SH \) (methyl thiol)
- \( CH_3OH \) (methanol)
- \( H_2O \) (water)
- \( C_2H_5OH \) (ethanol)

### Step 2: Determine Acidity Order
Next, we need to arrange these acids in order of acidity. The general trend is that thiols are more acidic than alcohols:
- \( CH_3SH > CH_3OH > H_2O > C_2H_5OH \)

### Step 3: Determine Basicity of Conjugate Bases
The basicity of the conjugate bases is inversely related to the acidity of their corresponding acids. Thus, the order of basicity (from strongest to weakest) for the conjugate bases will be:
- \( CH_3S^- \) (strongest base)
- \( CH_3O^- \)
- \( OH^- \)
- \( EtO^- \) (weakest base)

### Step 4: Relate Basicity to Nucleophilicity
Nucleophilicity generally increases with basicity, but there are exceptions based on the structure and the solvent. However, in this case, we can use the basicity order to infer nucleophilicity:
- \( CH_3O^- > CH_3S^- > OH^- > EtO^- \)

### Step 5: Final Order of Nucleophilicity
Thus, the decreasing order of nucleophilicity based on the above analysis is:
1. \( CH_3O^- \) (most nucleophilic)
2. \( CH_3S^- \)
3. \( OH^- \)
4. \( EtO^- \) (least nucleophilic)

### Conclusion
The correct order of decreasing nucleophilicity is:
**\( CH_3O^- > CH_3S^- > OH^- > EtO^- \)**