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- \(B\, <\, A\, <\, C\).