Maximum enol content is in:
Enol forms of both a and b exist. But the content is lower than the aldehyde & ketone terms respectively.
This is because the -c- bond is highly stable.
c) exhibits a stable enol form so its content will be more than a & b. This is because of chelation
The H bonding between O-H & -C- in the enol form further stabilizes the compound in addition to the conjugate alternative
5double-single-double bond in
d) also exhibits chelation enol content is even more than c owing to extended conjugate bonds (alternate single-double-single-double)