From http://support.apple.com/kb/HT5665 :
If you can’t reset them via the above instructions (you won't be able to add a rescue email address until you can answer your questions) then you will need to contact iTunes Support / Apple in your country to get the questions reset - which is likely to be by phone as they need to confirm your id and that it's your account.
Contacting Apple about account security : http://support.apple.com/kb/HT5699
When they've been reset (and if you don't already have a rescue email address) you can then use the steps half-way down this page to add a rescue email address for potential future use : http://support.apple.com/kb/HT5312