Which will have the highest enol content?

For a carbonyl compound the fraction present as its enol tautomer increases when the enol form enjoys extra stabilisation. Three effects are especially important:

(i) Resonance of the enolate/enol with more than one carbonyl group.
(ii) Intramolecular hydrogen bonding that locks the six-membered chelate ring.
(iii) Extended conjugation of the C=C bond formed in the enol.

Let us analyse each option in this light.

A simple aldehyde/ketone contains only one C=O group. The enol that could form is stabilised only by ordinary $$\pi$$-conjugation with that single carbonyl oxygen, so its mole-fraction is very small (typically <0.1 %). No intramolecular H-bonding is possible.

A simple ketone with no electron-withdrawing group on either side behaves almost exactly like Case A. The enol percentage is likewise very low.

Ethyl acetoacetate is a 1,3-dicarbonyl compound, so its enolate is resonance-stabilised by two carbonyls. However the ester carbonyl cannot engage in effective intramolecular hydrogen bonding with the -OH formed in the enol, and conjugation is interrupted by the intervening -O- atom of the ester. Hence the enol content is larger than in A or B but still moderate (≈7-8 %).

Pentane-2,4-dione (acetylacetone) is a true 1,3-diketone. In its enol form the molecule forms a six-membered ring via a strong intramolecular O-H···O hydrogen bond, and the $$\ce{C=C}$$ bond is conjugated simultaneously with both carbonyl groups:

$$\ce{CH3-C(=O)-CH2-C(=O)-CH3 \;\rightleftharpoons\; CH3-C(=O)-CH= C(OH)-CH3}$$

This dual resonance plus chelation lowers the energy of the enol dramatically, so the equilibrium lies far toward the enol side (≈70 % at room temperature).

Comparing all four, the stabilisation is greatest in Case D, therefore the enol content is the highest for that compound.

Option D which is: Pentane-2,4-dione (acetylacetone) has the highest enol content.