Question

when the switch is in off position, presence of which insulator obstructs the path of electric current in a circuit?

Answer 1

When the switch is off, no current will be flowing in the circuit... thus there is no significance of the insulator

Answer 2

When the switch is in the off condition, it is obvious that there will be no current passing through the same and therefore there is no need of any insulator to obstruct the path.

This condition applies only when the switch is in the on condition as only then the current passes through the circuit. So then only the insulator will be required.