It 's Doctor Who , anyone can come back , anything can be undone , if Rose can come back from another dimension , the Master can survive the Time War and the Daleks can keep escaping certain extinction ... they can get around this plot point should it ever serve them .

Question: what is the reason anyone can come back and anything can be done ?
Answer: None of the above choices .The reason is they are psychic .They have a contract to do this .Doctor who is a fictional story where time travel and almost anything else is possible .