If there is no possibly way to respond to a question, it's unanswerable. "What is the meaning of life?" is an unanswerable question.
Some questions are answerable, like "Who was the first person to walk on the moon?" and "What is your name?" Others are too complex or unknowable to be answered, like the unanswerable question, "Who is the happiest person in the world?" Both answerable and unanswerable trace their roots to the Old English answarian, "make an answer in reply."