It's a question without answer. / It's a question with no answer.
It's a question which has no answer. / It's a question which hasn't answer. / It's a question which doesn't have answer.
It's a question without AN answer / It's a question with no answer.
It's a question which has no answer / THIS ONE IS INCORRECT / It's a question which doesn't have AN answer.
These all mean the same thing. :)