What is the "time of great change" called

The correct answer is Progressive Era.

Explanation:

During the 19th century it happened that in the United States big businessmen and factories made a lot of money, while the population of workers did not have a good income while they had to work hard and in poor conditions.

This led to the emergence of unions that were fighting for workers' rights and fair pay for their jobs.

This caused many changes in the way of working in the United States, achieving fair hours and pay for all employees.

This "time of great changes" was called the progressive era.