Acidic character α stability of conjugate base (anion)
Stability of anion α−M and –I
−M>−I(dominance)
Electron withdrawing groups increase acidic strength. Among the given compounds, the electron withdrawing strength of substituents

In case of halogen −I>+M and −I order of halogen Cl>Br>I
Hence, CH(CN)3 will be most acidic