Among the following hydrogens which one is the most acidic?

\(H_a\)

The answer is 1. \(H_a\)

As we know acidic strength is directly proportional to the electon withdrawing capacity of the group. The electon withdrawing capacity of -NH is less than -C=O.

So, the conjugate base will be stabilized by two electon withdrawing groups and -C=O group has more electon withdrawing capacity and also by resonace. Therefore