The compound which cannot be used as a plasticizer, is

Options

A. di-n-butylphthalate
B. tricresyl phosphate
C. di-n-octylphthalate
D. diethyl phthalate

Explanation

To determine which compound cannot be used as a plasticizer, we need to analyze the options provided and understand the characteristics of plasticizers.

### Step-by-Step Solution:

1. **Understanding Plasticizers**:
- Plasticizers are substances added to materials to increase their plasticity and flexibility while reducing brittleness. They are typically high boiling esters.

2. **Evaluating Each Option**:
- **Option A: Di-n-butyl phthalate (DBP)**:
- This is a high boiling ester formed from n-butanol and phthalic anhydride. It is commonly used as a plasticizer. Therefore, this option can be used as a plasticizer.

- **Option B: Tricresyl phosphate**:
- This is a phosphate ester derived from phosphoric acid and alcohol. It is also a high boiling ester and is used as a plasticizer. Thus, this option can be used as a plasticizer.

- **Option C: Di-n-octyl phthalate**:
- This compound is another high boiling ester, similar to di-n-butyl phthalate, and is used as a plasticizer. Therefore, this option can also be used as a plasticizer.

- **Option D: Diethyl phthalate**:
- This compound is a low boiling ester, which means it has a lower molecular weight compared to the other options. Low boiling esters are typically not effective as plasticizers because they do not provide the necessary flexibility and plasticity. Thus, this option cannot be used as a plasticizer.

3. **Conclusion**:
- The compound that cannot be used as a plasticizer is **Diethyl phthalate** (Option D).

### Final Answer:
The compound which cannot be used as a plasticizer is **Diethyl phthalate**.

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