In the given molecule
- D's lone pair on oxygen is in conjugation with C=O group. Consequently it acquires a positive charge, its Kb is less for protonation to take place (as shown in scheme 1, as pointed by Mithoron).
- At C lone pair on this ring (furan) are delocalised and is AROMATIC (as shown in Scheme 2). Comparatively is kb value is less for protonation to take place.
- At B carbon of carbonyl carbon is sp2 hybridised and is more withdrawing then sp3 hybridised carbon (hybridisation is useful here). Hence due to greater inductive polarisation by carbonyl carbon lone pair on oxygen are donated less.
- At A, lone pair on oxygen is not in conjugation as in C and D. Further carbon bonded to alcohol at A is sp3 hybridised and is less withdrawing inductively then carbon at B which is sp2 hybridised.
Summing, OH group at A has highest Kb value. Its tendency to donate lone pair is comparatively more then B, C or D.
