The power of political machines declined during which period?

The correct answer is the progressive era.

Progressive era is the period when the power of political machines was declined.

There was political reforms and social activism in United states during this period. The main purpose for progressive movement was to eliminate problems which were being caused by immigration, urbanization, political corruption and industrialization.

This movement their aim was to target the bosses and the political machines.

The activists reformed public education, insurance, medicine, industry and finance.

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.