Acidic character of a species is defined on the basis of ease with which it can lose its H– atoms.
The hybridization state of carbon in the given compound is:
As the s–character increases, the electronegativity of carbon increases and the electrons of C–H bond pair lie closer to the carbon atom. As a result, partial positive charge of H– atom increases and H+ ions are set free.
The s–character increases in the order:
sp3 < sp2 < sp
Hence, the decreasing order of acidic behaviour is Ethyne > Benzene > Hexane.