hclo4 is the strongest acid
acidity of a molecule depends on its ability to lose H+
if a hclo4 loses H+ its conjugate base is formed clo4-
if stability of clo4- is more , hclo4 has more tendency to lose the H+
coming to the stability of clo4-
due resonance the clo4- is more stabilised
so hclo4 is the strongest