The decreasing order of strength of the bases, `OH^(-), NH_(2)^(-), H-C-=C^(-)` and `CH_(3)-CH_(2)^(-)`:

Options

A. `CH_(3)-CH_(2)^(-) gt NH_(2)^(-) gt H-C -=C^(-)gt OH^(-) `
B. `H-C-=C^(-) gt CH_(3)-CH_(2)^(-) gt NH_(2)^(-)gt OH^(-)`
C. `OH^(-) gt NH_(2)^(-) gt H-C-=C^(-) gt CH_(3)-CH_(2)`
D. `NH_(2)^(-) gt H-C-=C^(-) gt OH^(-) gt CH_(3)-CH_(2)`

Answer

To determine the decreasing order of strength of the bases among `OH^(-)`, `NH2^(-)`, `H-C≡C^(-)`, and `CH3-CH2^(-)`, we will analyze the stability of the negative charge on each species. The strength of a base is inversely proportional to the stability of the negative charge; the less stable the negative charge, the stronger the base.

### Step-by-Step Solution:

1. **Identify the Bases**: The bases given are:
- `OH^(-)` (hydroxide ion)
- `NH2^(-)` (amide ion)
- `H-C≡C^(-)` (acetylide ion)
- `CH3-CH2^(-)` (ethyl ion)

2. **Analyze Stability of Negative Charge**:
- **`OH^(-)`**: The negative charge is on oxygen, which is highly electronegative and can stabilize the negative charge well. Thus, it is relatively stable.
- **`NH2^(-)`**: The negative charge is on nitrogen, which is less electronegative than oxygen but more electronegative than carbon. Therefore, it is less stable than `OH^(-)`.
- **`H-C≡C^(-)`**: The negative charge is on a carbon atom that is part of a triple bond (sp hybridized), making it more electronegative than sp² or sp³ hybridized carbons. This makes it more stable than `NH2^(-)`.
- **`CH3-CH2^(-)`**: The negative charge is on a sp³ hybridized carbon, which is the least electronegative of all the atoms involved. This makes it the least stable.

3. **Rank the Stability of Negative Charges**:
- Most stable: `OH^(-)` (due to high electronegativity of oxygen)
- Second: `H-C≡C^(-)` (due to sp hybridization)
- Third: `NH2^(-)` (less stable than `H-C≡C^(-)`)
- Least stable: `CH3-CH2^(-)` (least electronegative carbon)

4. **Determine the Order of Basicity**:
- Since the strength of a base is inversely proportional to the stability of the negative charge, we reverse the stability order to find the basicity order:
- Most basic: `CH3-CH2^(-)` (least stable)
- Next: `NH2^(-)`
- Then: `H-C≡C^(-)`
- Least basic: `OH^(-)` (most stable)

5. **Final Order of Basicity**:
- The final order of strength of the bases from strongest to weakest is:
- `CH3-CH2^(-) > NH2^(-) > H-C≡C^(-) > OH^(-)`

### Conclusion:
The decreasing order of strength of the bases is:
`CH3-CH2^(-) > NH2^(-) > H-C≡C^(-) > OH^(-)`