Explanation:

Keto-enol tautomerism:
- Keto-enol tautomerism is a type of structural isomerism where a ketone (keto form) interconverts with an enol (enol form) through a proton transfer.
- This phenomenon occurs due to the presence of a hydrogen atom attached to an alpha carbon adjacent to a carbonyl group.

Options:
- a) C6H5-CHO: This compound is a benzaldehyde and does not exhibit keto-enol tautomerism as it lacks an alpha carbon with a hydrogen atom.
- b) C6H5COCH3: This compound is acetophenone and can undergo keto-enol tautomerism as it has an alpha carbon with a hydrogen atom.
- c) C6H5COCH2COCH3: This compound is a diketone and can exhibit keto-enol tautomerism since both carbonyl groups have adjacent alpha carbons with hydrogen atoms.
- d) Both b) and c) show keto-enol tautomerism.

Conclusion:
In conclusion, keto-enol tautomerism is observed in compounds that have alpha carbons with hydrogen atoms adjacent to carbonyl groups, enabling the interconversion between the keto and enol forms. Therefore, options b) and c) exhibit keto-enol tautomerism.

Option D is correct...