What was the name given to the time period in the US during the early 1900s, that aimed to fix the problems in US society (as a whole)?

Progressive Era

Explanation:

widespread social activism and political reform across the United States that spanned the 1890s to the 1920s.

c. the dred scott v. sandford case,

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering