Usually you will need to contact iTunes store support but this workaround may help:
-On the device/computer that is asking you for the security questions go to "settings", > "store" > tap the Apple ID and choose view"Apple ID" and sign in.
- Tap on payment information and add a credit/debit card of your preference then select "done", in the upper right corner
- Sign out and back into iTunes on the device by going to "settings"> "store" > tap the Apple ID and choose "sign-out" > Tap "sign -in" > "use existing Apple ID" and you should be asked to verify your security code for the credit/debit card and NOT the security questions.
- At this time you can remove the card by going back in to edit the payment info and selecting "none" as the card type then saving the changes by selecting "done". You should now be able to use your iTunes store credit without answering the security questions.