Final answer:
In the 1900 census, it was revealed that approximately two million children were working in the United States, signifying the extensive use of child labor at the time.
Step-by-step explanation:
The correct answer is c. 1900. The 1900 census revealed that approximately two million children were working across the US. The census indicated that child labor was a substantial socio-economic issue during that time. Industrial and agricultural work often included children in the workforce, with many working on family farms or in daunting industrial environments such as textile mills and coal mines. This labor was often out of necessity for family survival, particularly given the demands of a rapidly industrializing and urbanizing society. It was not until the Progressive Era and significant reform efforts that the plight of working children began to see improvement.