The decreasing nucleophilic order of the following compounds is:
i. `PhSO_(3)^(o-)`, ii. `C_(2)H_(5)SO_(3)^(o-)`, iii. `C_(2)H_(5)COO^(o-)`, iv. `overset(o-)(C)N`, v. `overset(o-)(O)H`

Options

A. `(v) gt (iv) gt (iii) gt (ii) gt (i)`
B. `(i) gt (ii) gt (iii) gt (iv) gt (v)`
C. `(iv) gt (v) gt (iii) gt (ii) gt (i)`
D. `(i) gt (ii) gt (iii) gt (v) gt (iv)`

Explanation

To determine the decreasing nucleophilic order of the given compounds, we will analyze the basic strength of each compound, as nucleophilic strength is often correlated with basic strength when the nucleophilic centers belong to the same period.

### Step-by-Step Solution:

1. **Identify the Nucleophilic Centers**:
- The nucleophilic centers in the compounds are:
- i. `PhSO3^(-)` (sulfonate ion)
- ii. `C2H5SO3^(-)` (ethyl sulfonate ion)
- iii. `C2H5COO^(-)` (carboxylate ion)
- iv. `C^(-)N` (cyanide ion)
- v. `OH^(-)` (hydroxide ion)

2. **Determine the Conjugate Acids**:
- For each compound, we will determine the conjugate acid by adding H+:
- i. `PhSO3^(-)` → Phenylsulfonic acid
- ii. `C2H5SO3^(-)` → Ethylsulfonic acid
- iii. `C2H5COO^(-)` → Propanoic acid
- iv. `C^(-)N` → Hydrogen cyanide (HCN)
- v. `OH^(-)` → Water (H2O)

3. **Establish the Acidic Strength**:
- The acidic strength of the conjugate acids can be compared:
- Sulfonic acids (like phenylsulfonic and ethylsulfonic) are stronger acids than carboxylic acids (like propanoic acid).
- Among the sulfonic acids, phenylsulfonic acid is stronger than ethylsulfonic acid due to the electron-withdrawing effect of the phenyl group.
- Carboxylic acids are weaker than sulfonic acids, and between them, propanoic acid is stronger than HCN and water.

4. **Order of Acidic Strength**:
- Based on the above analysis, the order of acidic strength (from strongest to weakest) is:
- Phenylsulfonic acid > Ethylsulfonic acid > Propanoic acid > HCN > Water

5. **Determine the Basic Strength**:
- The basic strength is the inverse of the acidic strength:
- Therefore, the order of basic strength (and thus nucleophilic strength) is:
- OH^(-) > C^(-)N > C2H5COO^(-) > C2H5SO3^(-) > PhSO3^(-)

6. **Final Nucleophilic Order**:
- Thus, the decreasing nucleophilic order of the compounds is:
- `OH^(-)` > `C^(-)N` > `C2H5COO^(-)` > `C2H5SO3^(-)` > `PhSO3^(-)`

### Conclusion:
The final decreasing nucleophilic order is:
- `OH^(-) > C^(-)N > C2H5COO^(-) > C2H5SO3^(-) > PhSO3^(-)`