Could find the mechanism in brief online. Still have few queries. What makes the D2O not attack the the central carbon (to which OH is attached) and choose other the carbon instead (both have a double bond). Again, in the 'repeat' part of mechanism, there's only one carbon with double-bond (carbonyl group). Why does D2O attack the second carbon again and not the carbonyl group one? I'm really confused.
Short answer - Electrphile seeks an electron rich site to attack
Explanation - D2O contains partial charges on the atoms because of the difference in the electronegativity of D and O. Oxygen being more electronegative than Deuterium has partial negative charge and D has partial positive charge.
In the enol form the carbon atom directly attached to Oxygen also has partial positive charge (difference in electronegativity O being more electronegative than C). So, the already partially positive charged Deuterium can't attack a site which also has electron deficiency. So, it attacks the carbon next to it which is slightly electron rich when Oxygen lone pairs attack the adjacent carbon.