The beauty of Frank R. Stockton’s “The Lady, or the Tiger?” is that the reader never does discover who, or what, is behind the door that the courtier opens at the end of the story. The princess does know which door the lady is behind, though, and even though she gives her lover a signal to open the right-hand door, the reader is still left with doubts. The woman behind one of the doors is a rival of the princess for her lover’s favors, and the princess can’t bear the thought of her marrying her lover.
So, the decision is left up to the reader. The author first wrote the story to generate discussion at a party in the 1880’s, and he did such a great job with it that it still causes plenty of discussion to this day.