aciidic h ??

well yeah pritish........i mistook as sulfonic group[1]...shud be [a]

i too agree with a) . a better stabilised base is formed.

dude, in a) the H is acidified by the stable resonating structure of its conjugate base and the presence of electron withdrawing carbonyl group nearby.

in b) no such conditions available , except the carbonyl group two carbon away.

in c) similarly just the -ve charge comes on the sulphur atom after the hydrogen leaves. this too is not enhanced by any other factor.

in d) the hydrogen is acidified by carbonyl group present. but its acidic character is also decreased by the lone pai of O atom present nearby as it acts as an electron releaser.

thus, clearly , ans. is a).

thankx...

and for any general ques ...what are main points to eb kept in mind ???

just check how stable is the conjugate base after the acidic hydrogen is released. more stable is the base, mre acidic is the hydrogen. also apply the ERG and EWG stabilisation concepts. thats all.

kkk...thx...
EWG will stabilise naa ???so it will be more acidic

ya, EWG will stabilise -ve charge of conjugate base.so it will be more acidic

rocky when we compare acidity of different compounds, we compare the stability of their conjugate bases. More stable the conjugate base, more acidic the compound. Now if the charge on conjugate base is dispersed, it is stable. This can occur through resonance or polar effects(inductive effect). Here the C=O group is electron withdrawing, and it disperses the negative charge on O^- atom of COOH's conjugate base. Thus COOH proton is most acidic here.
Also, a) is more acidic than d) because the conjugate base of d) is destabilised by +R effect of O.