Which of the following sentences best describes the Progressive Era?

• It was a period in which many social reforms occurred.
• It was a period of unchecked abuses by big business.
• It was a period of civil war and the end of slavery.
• It was a period of westward expansion under the idea of manifest destiny.

The best sentence that describes the Progressive Era is: "It was a period in which many social reforms occurred." To arrive at this answer, we can eliminate the other options by looking at the characteristics of the Progressive Era.

The Progressive Era, which took place in the late 19th century and early 20th century in the United States, was characterized by a push for social and political reforms. During this time, there were efforts to address issues such as poverty, child labor, women's suffrage, and corruption in government. The Progressive Era is often associated with the belief in the need for societal progress and improvement.

Option 2, "It was a period of unchecked abuses by big business," might describe the Gilded Age that preceded the Progressive Era. The Gilded Age was marked by significant industrialization and powerful business interests, often resulting in unfair business practices and the accumulation of wealth in the hands of a few.

Option 3, "It was a period of civil war and the end of slavery," refers to a crucial period in American history known as the Civil War era or Reconstruction, which took place in the mid-19th century. This period was characterized by the conflict between the Northern and Southern states, primarily over the issue of slavery.

Option 4, "It was a period of westward expansion under the idea of manifest destiny," describes the earlier period of westward expansion in the 19th century when the United States expanded its territory across the North American continent. This phase occurred before the Progressive Era.

Therefore, based on the provided options, the most accurate description of the Progressive Era is the first option, "It was a period in which many social reforms occurred."