Final answer:
The Industrial Revolution is the historical development most closely associated with a higher standard of living for the middle class, enabling new forms of wealth generation and improving living conditions.
Step-by-step explanation:
The Industrial Revolution was a significant historical development that is closely associated with a higher standard of living for the middle class. Unlike the rigid social structure of feudalism, which determined individuals' status by birth and land ownership, the Industrial Revolution introduced new ways to accrue wealth and improve social mobility. The rise of factory work and urbanization, combined with advancements from the Enlightenment in science and politics, enabled the production of goods on a mass scale and fostered the growth of a wealthy middle class distinct from the working class and the older nobility.
This period also witnessed the creation of an urban labor force and brought forth opportunities for the middle class to access improved living conditions, such as sanitation, affordable goods, and health care, despite the harsh realities facing the working class. The middle class was able to differentiate itself by adopting values like morality, sobriety, and cleanliness. As a result, the Industrial Revolution reshaped societal structures and paved the way for substantial changes in the standard of living.