I think the event delegate is not properly set. Follow the below steps.
1) Select the dropdown in design mode and click on properties.
2) Select the lightning bolt icon on top of the property sheet and click on the "SelectedIndexChanged" event.
3) Enter the method name "drpClick" you have written in the code behind to handle this event.
4) Click Save and build the app. Re-run. It should work now.
Hope this helps.