Keto - enol tautomerism is oberved in :

Options

A. `H_(5)C_(6)-overset(O)overset(||)(C)-CH_(2)-overset(O)overset(||)(C)-C_(6)H_(5)`
B. `H_(5)C_(6)-overset(O)overset(||)(C)-CH_(3)`
C. `H_(5)C_(6)-overset(O)overset(||)(C)-H`
D. Both (a) and (b)

Explanation

**Step-by-Step Solution:**

1. **Understanding Keto-Enol Tautomerism**:
- Keto-enol tautomerism is a chemical equilibrium between a keto form (with a carbonyl group, C=O) and an enol form (with an alcohol group, -OH, adjacent to a double bond).
- The presence of an alpha hydrogen (hydrogen atom on the carbon adjacent to the carbonyl group) is essential for this tautomerism to occur.

2. **Analyzing the Given Compounds**:
- We need to identify which compounds exhibit keto-enol tautomerism based on their structure.
- The compounds in question are likely to be organic compounds with carbonyl groups and alpha hydrogens.

3. **Example of Acetone**:
- Consider acetone (CH3-CO-CH3). It has two alpha hydrogens on the methyl groups adjacent to the carbonyl carbon.
- The keto form is CH3-CO-CH3, and the enol form is CH2=CHOH. The tautomerism occurs when the alpha hydrogen is abstracted, leading to the formation of the enol.

4. **Evaluating Each Option**:
- **Option A**: C6H5C(=O)CH3 (acetophenone) has an alpha hydrogen and can undergo tautomerism.
- **Option B**: C6H5C(=O)C2H5 (another ketone) also has alpha hydrogens and can undergo tautomerism.
- **Option C**: A cyclic compound with a carbonyl group but no alpha hydrogen cannot undergo tautomerism.
- **Option D**: A compound without an alpha hydrogen will not exhibit keto-enol tautomerism.

5. **Conclusion**:
- Based on the analysis, options A and B exhibit keto-enol tautomerism due to the presence of alpha hydrogens, while options C and D do not.

**Final Answer**: Keto-enol tautomerism is observed in options A and B.

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