Among the following the weakest base is .

Options

A. a. `C_6H_5CH_2NH_2`
B. b. `C_6H_5CH_2NHCH_3`
C. c. `O_2NCH_2NH_2`
D. d. `CH_3NHCHO`

Explanation

To determine which among the given options is the weakest base, we need to analyze the structures of the compounds provided and the nature of their lone pairs of electrons. Basicity is defined by the ability of a compound to donate a lone pair of electrons. The more delocalized the lone pair is, the weaker the base will be, as it cannot readily donate the electrons.

### Step-by-Step Solution:

1. **Identify the Structures**:
- Let's denote the options as follows:
- Option A: C6H5-CH2-NH2
- Option B: C6H5-CH2-N(CH3)-H
- Option C: C6H5-CH2-NH2-NO2
- Option D: CH3-NH-C(=O)-H

2. **Analyze Option A (C6H5-CH2-NH2)**:
- The nitrogen atom has a lone pair that is localized. This means that the lone pair is available for donation, making this a relatively stronger base.

3. **Analyze Option B (C6H5-CH2-N(CH3)-H)**:
- In this case, the nitrogen is attached to a phenyl group (C6H5) and a methyl group (CH3). The lone pair on nitrogen is somewhat delocalized due to the presence of the phenyl group, but it is not as significantly affected as in other options. Therefore, it is still a moderate base.

4. **Analyze Option C (C6H5-CH2-NH2-NO2)**:
- Here, the NO2 group is a strong electron-withdrawing group. It pulls electron density away from the nitrogen, which decreases the availability of the lone pair for donation. This makes it a weaker base compared to options A and B.

5. **Analyze Option D (CH3-NH-C(=O)-H)**:
- In this structure, the carbonyl (C=O) group is also an electron-withdrawing group. The lone pair on nitrogen is delocalized into the carbonyl group, which significantly reduces its availability for donation. This makes it the weakest base among all the options.

6. **Conclusion**:
- After analyzing all options, we conclude that **Option D (CH3-NH-C(=O)-H)** is the weakest base due to the strong electron-withdrawing effect of the carbonyl group, which delocalizes the lone pair on nitrogen.

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