Selected bond angles for six hydrocarbons are shown below. Arrange these hydrocarbons according to their pK_a values, from the lowest to the highest.

 

most acidic because ring is aromatic after deprotonation.
$\mathrm{H}-\mathrm{C}\equiv \mathrm{C}-\mathrm{H}$ is next most acidic compound because %s characters are high.
 

is least acidic because anion is anti aromatic and remaining compounds are arrange on the basis of % s characters.