Which has the highest enol content?
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.


 


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.