No one is hooking you up to a lie detector here. With that said, my answer here, as would be my approach to the interview in general, is to 'sell the best you'. So tell the truth but present it in a way that makes you look good.
So what are your reasons for thinking that you're about to get the axe? If you don't ask yourself that question, you'll be caught out when the interviewer inevitably asks the same.
If it is performance or culture related, you could say something like "I came to realise after a month that I wasn't a good fit for them/their corporate culture".
If it's redundancy or layoffs you are worried about, you can usually explain that as-is or you can say something like "I became concerned about the direction the company is taking with regards to my role".
Think about the real reason, then salesman/woman it up.