un ‧ an ‧ swer ‧ a ‧ ble /ʌnˈɑːns ə rəb ə l $ ʌnˈæn-/ BrE AmE adjective
1 . an unanswerable question is one that cannot be answered
2 . definitely true and therefore impossible to argue against:
The case (=reasons for doing something) for better public transport is unanswerable.