If something is legally permissible, then its either permissible because the law explicitly says so or there is no law stating it is impermissible.
So "A" must be false because if you believe that it is impossible for something legally permissible to be immoral then you can't also believe that the law does not cover all circumstances in which one morally wrongs another.
Why?
(Because) If you believe that the law does not cover all circumstances in which one morally wrongs another, then it is possible that something that is legally permissible (by virtue of the fact that there is no law stating it is impermissible) is immoral.
Comments
If something is legally permissible, then its either permissible because the law explicitly says so or there is no law stating it is impermissible.
So "A" must be false because if you believe that it is impossible for something legally permissible to be immoral then you can't also believe that the law does not cover all circumstances in which one morally wrongs another.
Why?
(Because) If you believe that the law does not cover all circumstances in which one morally wrongs another, then it is possible that something that is legally permissible (by virtue of the fact that there is no law stating it is impermissible) is immoral.