Which of the following compounds does NOT exhibit keto-enol...

Keto-enol tautomerism is exhibited by carbonyl compounds that have at least one $\alpha$-hydrogen atom.

In $\mathrm{CH_3COCH_3}$, there are six $\alpha$-hydrogen atoms.

In $\mathrm{C_6H_5COCH_3}$, there are three $\alpha$-hydrogen atoms.

In $\mathrm{C_6H_5COC_6H_5}$ (benzophenone), the carbonyl carbon is attached to two phenyl rings. The carbon atoms of the phenyl rings adjacent to the carbonyl group do not have any hydrogen atoms. Since there are no $\alpha$-hydrogen atoms, it does not exhibit keto-enol tautomerism.

In $\mathrm{CH_3CH_2CHO}$, there are two $\alpha$-hydrogen atoms.

Answer: $\mathrm{C_6H_5COC_6H_5}$