I am not an expert but I try to help you
In "Activity_Pause" event, if you don't write any code, the activity close and "return" to the activity that has call it,
so if Opt1_Activity1 call OptionSelect, it's normal that when the customer press backKey, it "return" to Opt1_Activity1.
If you want to "return" to a wanted activity, you need to insert this code into Activity_Pause event on OptionSelect activity:
or StartActivity("ItemList"), I don't remember the exact words, try it
Sergio