Hi there @Joe_dog,
Welcome back to our Community - we are sorry to understand that you're still having issues registering for your online account. Is this for your Mobile account as per your last thread to us?
https://community.virginmedia.com/t5/Virgin-Mobile/Registering-a-new-account-account-been-asked-for-...
As advised on that thread, the security answer is the same answer that you provide when you speak to us by phone and provide a password to verify your identity as the account holder.
If you're still having issues when trying that, please feel free to reply either on this thread or your initial thread.
Cheers