The correct answer is A. The Chinese Exclusion Act restricted immigration solely on the basis of a person's race.
The Chinese Exclusion Act was a US federal law passed on May 6, 1882, that allowed government to suspend the immigration of Chinese nationals, with Congress taking the necessary measures for its rapid implementation. The law remained in force for ten years, before being renewed, and then extended to the Japanese.
This was the first significant restriction on free immigration in the United States.