The extent of enol content can be explained on the basis of acidic hydrogen, intramolecular hydrogen bonding.
- The stability of the carbanion formed is dependent on its stability after it is formed. In compound $\ce{(c)}$, the carbanion is stabilized by extended conjugation due to presence of phenyl group. Such stabilization does not occur in $\ce{(d)}$. In $\ce{(d)}$, methyl substituent at carbanion increases negative charge via inductive polarization. The resulting carbanion is unstable.
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After enolization , enol is stabilized via intramolecular hydrogen bonding, which is a common feature in all examples provided.
The enol in compound $\ce{(c)}$ is stabilized via extended conjugation.
Taking the above factors into account, $\ce{(c)}$ has highest enol content.
