During the industrial revolution, most "rags to riches" stories occurred in the middle class due to

The correct answer is industrialization

Industrialization is the modernization process through which a society's means of production go. It is accompanied by technological expansion and economic development.

Industrialization is an old process in humanity. Still during the Middle Ages, new techniques marked the advance of the means of production and productivity. But that does not mean that there were industries as we know them today or characteristics of capitalism. Progress has gone through several technological phases. More improved techniques in agriculture, crafts and manufacturing have contributed to the full development of the industry.