This article explains why you may be unable to use your security questions to reset or change your password.   

In an effort to prevent accounts from being compromised, we've introduced a feature that prevents your secret questions from being used to obtain a password using the online password reset flow. This is a protection measure designed to keep your Yahoo! account secure. This feature is enabled when you haven't accessed your account recently and an attempt to receive a password using the secret questions is performed. 

If you no longer have access to the alternate email address or the mobile number on file, you can request to use your secret questions within our online password reset flow. Once requested, you can attempt to verify your secret questions in 24 hours. 

If you did not make an attempt to reset the password using your security questions, we recommend changing your security questions and password. Also, we strongly advise visiting the Yahoo! Security Center to learn more about account security, and ways you can safeguard your account.

NOTE: If you are unable to use your security questions to reset your password after 24 hours, you will need to contact Yahoo! Customer Care for further assistance.