Which of the following hydrocarbons is the strongest Bronsted acid?

The acidity of a C-H bond increases with the s-character of the carbon, because the conjugate base holds its lone pair closer to the nucleus. Ethyne carbon is $sp$ (50% s), ethene and benzene carbons are $s{p}^2$ (33% s), and ethane carbon is $s{p}^3$ (25% s). So ethyne is the strongest acid.