The correct order of acid strength is
HClO4 < HClO3 < HClO2 < HClO
HClO < HClO2 < HClO3 < HClO4
HClO4 < HClO < HClO2 < HClO3
HClO2 < HClO3 < HClO4 < HClO
Acidic nature and thermal stability of oxy acids increases with increasing oxidation state of chlorine :
HClO < HClO2 < HClO3 < HClO4.