Filling out an application, don't know what to write

Why did you leave? Did you get fired, did you quit and why, only you can answer this because no one knows why you left your job, only you know.

Just be honest because the prospective employer will call your previous employers and ask them. This will show if you were truthful or not on your application.

Just answer the question honestly. If you quit to take another job or got fired, just say so. It's better to be up front than to get caught up in it later.