Among the given compounds, the most susceptible to nucleophilic attack at the carbonyl group is:

Options

A. `CH_(3)CH_(2)CH_(2)CH_(2)COOC_(2)H_(5)`
B. `C_(6)H_(5)COOC_(2)H_(5)`
C. `C_(6)H_(11) CH_(2)COOC_(2)H_(5)`
D. `C_(6)H_(5)CH_(2)COOC_(2)H_(5)`

Answer

To determine which compound is most susceptible to nucleophilic attack at the carbonyl group, we need to analyze the structures of the given compounds and assess the factors that influence nucleophilic attack, particularly the charge density at the carbonyl carbon.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We have four compounds to analyze. Let's denote them as:
- Compound A: C4H9C11O C2H5
- Compound B: C6H5C(=O)C2H5
- Compound C: C6H11CH2C(=O)C2H5
- Compound D: C6H5CH2C(=O)C2H5

2. **Understand Nucleophilic Attack**:
Nucleophilic attack occurs at the carbonyl carbon (C=O) where the carbon is electrophilic due to the partial positive charge created by the electronegative oxygen. The susceptibility to nucleophilic attack is influenced by the electron-withdrawing or donating effects of substituents attached to the carbonyl carbon.

3. **Analyze Resonance Effects**:
- In Compound A, the carbonyl carbon is bonded to a C4H9 group and a C2H5 group, which are both electron-donating alkyl groups. This may increase electron density at the carbonyl, but not significantly.
- In Compound B, the carbonyl is adjacent to a phenyl group (C6H5). The resonance from the phenyl group can delocalize the electron density away from the carbonyl carbon, reducing its susceptibility to nucleophilic attack.
- In Compound C, the carbonyl is attached to a C6H11 group and a C2H5 group. Similar to Compound A, these groups are electron-donating, but the presence of the longer alkyl chain may not significantly enhance the electrophilicity of the carbonyl.
- In Compound D, the carbonyl is again adjacent to a phenyl group, which will also delocalize electron density away from the carbonyl carbon.

4. **Evaluate Charge Density**:
The key factor is the charge density at the carbonyl carbon. The more positive charge density (or less electron density) at the carbonyl carbon, the more susceptible it is to nucleophilic attack.
- Compound B has a significant resonance effect from the phenyl group that reduces the positive charge density at the carbonyl carbon.
- Compounds A, C, and D have alkyl groups that do not significantly reduce the charge density at the carbonyl.

5. **Conclusion**:
Based on the analysis, Compound B is most susceptible to nucleophilic attack at the carbonyl group due to the resonance effect from the phenyl group, which reduces electron density at the carbonyl carbon.

### Final Answer:
The most susceptible compound to nucleophilic attack at the carbonyl group is **Compound B** (C6H5C(=O)C2H5).