When an ending to a story is unclear or uncertain, it's ambiguous. Option B is correct.
By beeing ambiguous, the ending of the story may be interpreted in different ways by the reader´s point of view.
An ambiguous ending, that is, and ending of a story that is not pretty clear, invites the reader to create his o her own opinion, and makes them arrive at his o her own conclusions based on the facts and events that were described and developed throught the whole story.