Why was the Hatbox Ghost removed from The Haunted Mansion?
The Answer is: B!
While urban legends claimed that both A & C were true, the real reason why the Hatbox Ghost was removed was because the effect never looked realistic and Imagineering didn't have time to work on improving it.
As most guests know, the ghosts in the Haunted Mansion are more treat than trick, so nobody has ever had a fright related heart attack inside the building. While Sharon Tate did get murdered the night before the Haunted Mansion opened, the tragic event never had an effect on the operation of the attraction.