Why did the United States adopt the policy of hypodescent?

Why did the United States adopt the policy of hypodescent?
 
  a. By conflating the cultural and biological, the United States used this policy historically to discriminate.
 b. This policy was primarily in place as a way to encourage families to produce more children across perceived racial lines.
  c. Primarily applied to limit economic transactions across racial groups, it, instead, served to create a caste system in the United States.
  d. Hypodescent was used during the epoch of slavery as a way to prevent social and reproductive interaction between the races.