This can be explained on the basis of conjugation as well as electronegativity,
(i) In the `prop`, `beta` unsaturated acid, the carbanion is stabilised by resonance or conjugation effect. This is not possible in `CH_(3)CH_(2)COOh`.
(ii) In the unsaturated acid, the carbon atom to which carboxyl group is attached is more electronegative `(sp^(2)` hybridised) as compared to the carbon atom `(sp^(3) hybridised)` in the other acid. Therefore, the release of proton `(H^(+))` from unsaturated acid is easier and it is therefore, a stronger acid.
