(A) One tag per campaign
(B) One tag per Ad Group
(C) One tag per Account
(D) One tag per website
The best practice for UET tags is using One tag per website. If you use the same tag for multiple websites then there may be some problems like If the same tag is used for multiple websites, it is possible that only one of the websites has been properly installed with a UET tag but you won’t be able to tell since Microsoft Advertising will mark the status of the UET tag as active.
This question is a part of the Microsoft Advertising Certified Professional exam. You can find all the answers with detailed explanation for this exam in the Microsoft Advertising Certified Professional exam page.