Final answer:
The middle class was most likely to join reform movements, focusing on morality and social issues, while the working class engaged through unions and political action. Farmers also played a role initiating Populism.
Step-by-step explanation:
The group most likely to join reform movements during the times of industrialization and urbanization were primarily the middle class. Middle-class reformers were significant in their efforts to improve society by focusing on moral behavior and advocating for issues such as temperance and women's rights. Another key group were the members of the working class, who sought to improve their lives by forming unions and turning to political action to protect their interests.
In the context of the Populist movement, it was the farming class who initiated the idea and attempted to create alliances with urban workers to form the People's Party. Their influence, however, faded by the 1900s as the major political parties absorbed some of their ideas during the Progressive Era.