Okay. So my cute little happy and unhappy facces came through only as question marks. Sorry about that. Here it is again.
Once you say “the reason,” all other words implying the reason are unnecessary. These are always wrong.
…The reason why is that she was ill.
…The reason is because she is ill. [This one leads the list!] …The reason is on account of she was ill.
…The reason is due to the fact she is ill.
These are the ONLY correct ways to say this.
…The reason is that she was ill.
…The reason is she was ill.