Where is the Security Question and Answer?

In v7.x and earlier the Security Question and Answer was displayed on the Client's Profile tab:

v7.x Profile tab

In v8.0 and above it is now located under the Users tab and accessed by clicking the down arrow next to the Manage User button:

v8.x Users tab

More information on the new functionality on this tab is  published in the documentation: https://docs.whmcs.com/Clients:Users_Tab