Final answer:
Selective placement by adoption agencies can complicate adoption studies by making it difficult to separate genetic factors from environmental influences, as agencies might match children with adoptive parents based on similar traits or backgrounds.
Step-by-step explanation:
The practice of adoption agencies that can make adoption studies problematic for researchers interested in disentangling genetic and environmental contributions to human behavior is c. selective placement. Selective placement occurs when adoption agencies match children to adoptive parents based on similarities in traits, behaviors, or background, which can obscure the separation of genetic factors from environmental influences.
The reason this practice can be problematic is that it can create a situation where the placed child's environment is not independent of their biology, making it harder for researchers to determine the degree to which temperament and innate traits are influenced by genetics versus the upbringing environment.