Which carbonyl group is protonated most readily in acidic solution, but gives nucleophilic addition by `NH_4NH-Ph` least readily ?

Options

A.
B.
C.
D.

Explanation

To solve the question, we need to analyze the stability of the carbocation formed upon protonation of different carbonyl groups and their reactivity towards nucleophilic addition by phenylhydrazine (NH4-NHPh).

### Step-by-Step Solution:

1. **Identify the Carbonyl Groups**:
We need to consider the different carbonyl groups presented in the options. Let's denote them as A, B, C, and D for simplicity.

2. **Protonation of Carbonyl Groups**:
When a carbonyl group (C=O) is protonated in an acidic solution, it forms a more electrophilic species. The stability of the resulting carbocation (formed after protonation) is crucial for determining which carbonyl group is protonated most readily.

3. **Evaluate Carbocation Stability**:
- **Option A**: Contains a benzene ring. The carbocation formed here is moderately stable due to resonance.
- **Option B**: Contains a methyl group (CH3). The +I effect from the methyl group stabilizes the carbocation, making it more stable than Option A.
- **Option C**: Contains an O-Me (methoxy) group. The +M effect from the lone pair on the oxygen significantly stabilizes the carbocation, making it the most stable among the options.
- **Option D**: Contains a nitro group (NO2). The -M effect from the nitro group destabilizes the carbocation, making it the least stable.

4. **Conclusion on Protonation**:
Based on the stability of the carbocations formed, Option C (with the O-Me group) is protonated most readily due to the high stability of the carbocation formed.

5. **Nucleophilic Addition Reaction**:
Now, we need to consider the nucleophilic addition of NH4-NHPh to these carbonyl groups. The more electron-dense the carbonyl carbon is, the less readily it will undergo nucleophilic attack.
- In Option C, the presence of the O-Me group increases electron density at the carbonyl carbon due to the +M effect, making it less susceptible to nucleophilic attack.

6. **Final Answer**:
Therefore, the carbonyl group that is protonated most readily (Option C) also gives nucleophilic addition by NH4-NHPh the least readily.

### Final Answer:
**Option C** is the carbonyl group that is protonated most readily in acidic solution but gives nucleophilic addition by NH4-NHPh least readily.