We ask a challenge question when we detect that you are trying to logon from a new computer or different location. This is to prevent someone with stolen passwords from logging into your account. Since only you know the answer to the questions, we will know it’s really you. Generally you will be asked to answer a secret question only when you logon for the first time from a new computer or at a new location.
After you answer the question, you will be asked whether we should register this computer for future logons. If you are using a personal computer, you should register it. If you are using a public computer, you do not want to register it.
If you register your computer, and continue getting asked to answer the security question, you may have selected settings in your browser that automatically delete cookies.