Which of the following has least `pk_(b)`, value?

Options

A.
B.
C.
D.

Explanation

To determine which of the given compounds has the least \( pK_b \) value, we need to understand the relationship between \( pK_b \), basicity, and the presence of electron-donating groups.

### Step-by-Step Solution:

1. **Understanding \( pK_b \)**:
- \( pK_b \) is a measure of the basicity of a compound. The lower the \( pK_b \) value, the stronger the base. Thus, we are looking for the compound with the least \( pK_b \).

2. **Identifying Electron-Donating Groups**:
- Basicity is enhanced by the presence of electron-donating groups (EDGs). These groups increase the electron density on the nitrogen atom (or the atom that is accepting protons), making it more likely to accept protons (H⁺ ions).

3. **Analyzing the Compounds**:
- We need to examine the compounds given in the options and count the number of electron-donating groups (like \( CH_3 \)) attached to them.
- More \( CH_3 \) groups or other EDGs will lead to a higher \( pK_b \) value, which means a weaker base.

4. **Comparing the Compounds**:
- For each compound, count the number of \( CH_3 \) groups:
- **Compound A**: 2 \( CH_3 \) groups
- **Compound B**: 4 \( CH_3 \) groups
- **Compound C**: 3 \( CH_3 \) groups
- **Compound D**: 4 \( CH_3 \) groups
- The compound with the least number of \( CH_3 \) groups will have the least \( pK_b \) value.

5. **Conclusion**:
- Since Compound A has the least number of \( CH_3 \) groups (only 2), it will have the least \( pK_b \) value among the options provided.

### Final Answer:
- The compound with the least \( pK_b \) value is **Compound A**.