Freemasonry
Freemasonry, as it exists in various forms all over the world, has a membership estimated by the United Grand Lodge of England at around six million worldwide. The fraternity is administratively organised into independent Grand Lodges (or sometimes Grand Orients), each of which governs its own Masonic jurisdiction, which consists of subordinate (or constituent) Lodges. The largest single jurisdiction, in terms of membership, is the United Grand Lodge of England (with a membership estimated at around a quarter million). The Grand Lodge of Scotland and Grand Lodge of Ireland (taken together) have approximately 150,000 members. In the United States total membership is just under two million.

Q: What is the largest single jurisdiction of Freemasonry?
United Grand Lodge of England