It seems that my clients’ Yahoo accounts get hacked more than other series.

First of all, turn on two step (two factor) authentication. This is much more secure than the (easily guessed) security questions. This will require a text message to your cell phone to log into the Yahoo account from a new computer or device.

You can also view the location history for logins to your Yahoo account. Follow these steps. If you notice a login from a strange place. That is a cause for concern.

https://help.yahoo.com/kb/check-recently-accessed-account-sln2073.html?impressions=true