The hydrogen atoms of ethyne are most acidic

The hydrogen in benzene is least acidic as it is electron rich and hence there is no way for hydrogen to lose electron. 
In the rest rest three compounds, hydrogen gains partial positive charge by donating electron density to the carbon's pi orbital. Since more pi orbitals are there in ethyne, most electron density is lost by hydrogen of ethyne and hence it acquires positive charge and leave as H+ ion. Hence it is most acidic.