Which out of ethylene & acetylene is more acidic and why ?

Explanation

Greater the s-character of a hybridised carbon atom, greater will be its electronegativity. The s-character of sp-hybridised carbon atom of acetylene is greater than that of the `sp^2`-hybridised carbon atom of ethylene. So electronegativity of the carbon atom of acetylene `(CH-=CH)` is greater than the carbon atom of ethylene `(CH_2=CH_2)` .
The tendency of H-atom attached to the more electronegative carbon atom to be removed as proton `(H^+)` is relatively higher. So, acidity of ethylene is less than that of acetylene i.e., acetylene is more acidic than ethylene.