How to determine the strongest oxyacid of chlorine?
In such questions you're supposed to know certain rules of acidity. You can find some of them on this page. In your case fourth rule, concerning oxoacids, is relevant and as you can see there, it's described on this very case. More precise formulation is so called Pauling's Rule described here. In general the more oxo groups connected to central atom, the more acidic compound.

Related: How are Pauling&#39;s rules applied to Nitric acid?