c
The greater $I$ effect is favorable or greater acidic strength. The $-I$ effect of aldehyde group is greater than the ester group. Due to this $C$ will be most acidic. In $A$ and $B , B$ will be less acidic as it has $-I$ effect sort of localized due to ester group being on a single carbon. In $A$ it is more delocalized is not effective on more by adjacent carbons.
Hence, the required order is- $B\, <\, A\, <\, C$.