\Lambdacetylenic hydrogens are acidic because
(1) sigma electron density of $\mathrm{C}-\mathrm{H}$ bond in acetylene is nearer to carbon which has $50 \%$ s-character
(2) acetylene has only one hydrogen on each carbon
(3) acetylene contains least number of hydrogen among the possible hydrocarbons having two carbons
(4) acetylene resembles acetic acid