Which of the following has highest % enol content in the liquid?

The correct option is D.
A very common form of triad tautomerism in a carbonyl compound between a >C=O group and an alpha H where the equilibrium lies between a keto form and an enol form is known as keto-enol tautomerism.

Option (b) has a strong active methylene group and the enol form is more stable due to the presence of a stable aromatic ring.

Trick:

See hydrogen bond formation along with ring size, if more hydrogen bond enol is more stable.