I have insufficient total answer score on this tag, but to me it seems straightforward that AD and Active Directory are one and the same. Yet I can't suggest that they be made synonyms of each other.
I read this Answer but I still can't submit it, due to insufficient points on the AD or Active Directory tags. The system probably doesn't work that well if nobody else has made the connection between these two tags yet. Just putting it out there for discussion...
Further observation - the a year in Moderation 2018 post shows only 4 Tag Synonyms were proposed in 2018 - not very many! Are the experience requirements too strict? Yet I do appreciate the other side of this, that SPSE wish to uphold the tag 'data quality'.
Year 2020 update - the year in Moderation 2019 post show 6 Tag Synonyms were proposed in 2019 - a 50% increase, woo-hoo! Still nobody has marked AD and Active Directory as tag synonyms. I realise it's not the most exciting topic to post about, but it's curious that it receives no attention. I still can not even suggest the two tags as synonyms...