Which of the following has the highest enol content at equilibrium?

The enol of pentane-2,4-dione is stabilised by conjugation of $\mathrm{C}=\mathrm{C}$ with $\mathrm{C}=\mathrm{O}$ and by a strong intramolecular hydrogen bond. Simple monocarbonyl compounds such as propanone, ethanal and ethyl ethanoate have negligible enol content. So pentane-2,4-dione is highest.