Who were the “other half” that Jacob Riis referred to in his 1890 book How the Other Half Lives? A the factory owners
B the upper class
C the working poor
D the middle class

the working poor

Explanation:

Jacob A. Riis was a journalist and he was also a social reformer. In the 19th century, he publicized the challenges that people faced with regards to housing and poverty.

He used his writing to expose the hardship that immigrants faced and the plight and poverty among them. He took photograph of their saloons, slums, etc that the working poor in New New York were faced with.

The other half” that Jacob Riis referred to in his 1890 book How the Other Half Lives were the working poor.

a and b because with the other three, they simplify to something else.