Answer: b. WithClientSecret
Explanation: This method sets the application secret used to prove the identity of the application when requesting tokens from Azure Active Directory. WithCertificate is used to set the certificate that is used for the app to authenticate with Azure AD. WithClientId is used to set the client ID of the application, WithRedirectUri is used to set the redirect URI of the application and WithAuthority is used to set the authority to be used for the app's authentication.