Earlier quoted context omitted.
The choice with Mormons is technically correct. Those who use theological considerations to define Christianity generally don't accept Mormons as Christians. For example, the Catholic Church sees Mormonism a non-Christian religion, because it rejects the Nicene Creed. People who base Christianity on cultural identity are more likely to include Mormons in the definition.
It does seem to be common to make LDS not counted as Christianity, although the reason is unclear to me. Another comment mentions that the rejection of the Nicene Creed does not seem to be the distinction. I had thought that it is because they have the Book of Mormon, although that is unclear. Orthodox have additional books of the Bible that Catholics do not have, but are still Christian. (Although, I think the addit…
All mainstream Christian denominations are branches of that original church. Some may see each other heretical, but they still acknowledge the common heritage. If you now have a religion that takes some aspects of Christianity, adds something of its own, and rejects the part all those schismatic branches agree on, it does not look like you are a branch of that "one holy catholic apostolic church".
The same applies to Jehovah's Witnesses.