Among the following the weakest base is .

Options

A. `C_(6)H_(5)CH_(2)NH_(2)`
B. `C_(6)H_(5)CH_(2)NHCH_(3)`
C. `O_(2)NCH_(2)NH_(2)`
D. `CH_(3)NHCHO`

Explanation

To determine which of the given amines is the weakest base, we need to analyze the basicity of each compound based on the effects of substituents attached to the nitrogen atom. Basicity in amines is primarily influenced by the availability of the lone pair of electrons on the nitrogen for protonation.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the four amines as A, B, C, and D. Each amine has a nitrogen atom with a lone pair of electrons.

2. **Analyze Inductive Effects**:
- **Compound A**: It has electron-withdrawing groups (EWGs) attached to the nitrogen. The presence of these groups will pull electron density away from the nitrogen, reducing the availability of the lone pair for bonding with protons. This leads to decreased basicity.
- **Compound B**: This compound has both electron-withdrawing and electron-donating groups. The electron-withdrawing group will decrease basicity, but the electron-donating group will increase it. The overall effect will depend on the strength of these groups.
- **Compound C**: Similar to Compound A, this compound has a strong electron-withdrawing group (like NO2). This will significantly reduce the basicity due to the strong -I effect.
- **Compound D**: This compound has a resonance effect due to the presence of a carbonyl group (C=O). The lone pair on nitrogen can participate in resonance with the carbonyl, making it less available for protonation, thus reducing its basicity.

3. **Compare Basicity**:
- **Compound A**: Weak base due to -I effect.
- **Compound B**: Moderate base due to a balance of -I and +I effects.
- **Compound C**: Very weak base due to strong -I effect from NO2.
- **Compound D**: The weakest base due to resonance effect, which significantly delocalizes the lone pair on nitrogen.

4. **Conclusion**:
Based on the above analysis, **Compound D** is the weakest base among the four options provided. The resonance effect significantly reduces the availability of the lone pair for protonation, making it less basic compared to the others.

### Final Answer:
The weakest base among the given options is **Compound D**.