The rise of the middle class can predominantly be attributed to the Industrial Revolution. The Industrial Revolution led to significant economic changes, including the growth of industries and urbanization, which created new jobs and opportunities for people to ascend to middle-class status.
The other options you mentioned, such as state policy to have three different classes, publicization of farmland, and improved conditions of the lower class, may have played a role in societal changes, but they are not the primary drivers of the rise of the middle class in the same way that the Industrial Revolution was.
In summary, the best answer is the Industrial Revolution.