A. Load all country and state combinations in a data table and write a Constraint rule to lookup the data table.

B. Create a Constraint rule for each country in the drop-down list by using Simple Conditions and select valid states for the Action attribute of the rule. In this case, number of rules will be equal to the number of countries in the list.

C. Create Hiding rules for each country with Simple Conditions, and then select valid states for the Action attribute. In this case, the number of Hiding rules will be equal to the number of countries.

D. Use the Hiding rule, which is the only possible option, because arrays do not do not support Constraint rules.