The correct order of acid strength is:

• A

  $\mathrm{HClO}_4 < \mathrm{HClO}_3 < \mathrm{HClO}_2 < \mathrm{HClO}$
• Correct answer
  B

  $\mathrm{HClO} < \mathrm{HClO}_2 < \mathrm{HClO}_3 < \mathrm{HClO}_4$
• C

  $\mathrm{HClO}_4 < \mathrm{HClO}^2 < \mathrm{HClO}_2 < \mathrm{HClO}_3$
• D

  $\mathrm{HClO}_2 < \mathrm{HClO}_3 < \mathrm{HClO}_4 < \mathrm{HClO}$

Solution

Oxygen is more electronegative than chlorine. With an increase in the number of $\mathrm{O}$ atoms attached to $\mathrm{Cl}$, more electrons are pulled away from $\mathrm{O}-\mathrm{H}$ bond and weaker becomes the $\mathrm{O}-\mathrm{H}$ bond. This increases the acid strength. The decreasing order of acidic strength is $\mathrm{HClO} < \mathrm{HClO}_2 < $ $\mathrm{HClO}_3 < \mathrm{HClO}_4$

Hence, the correct answer is option: 'B'.