What practice of adoption agencies can make adoption studies problematic for researchers interested in disentangling genetic and environmental contributions to human behavior?

a. open adoption

b. environmental grooming

c. selective placement

d. sealed records

The correct answer is option d. "sealed records".

Explanation:

Some adoption agencies give the option that biologically and adopting parents prevent to find, or even know, each other. This practice is known as "sealed records" and support the biological parents to give their children to adoption and avoid any possible criticism for their decision. However, this practice make adoption studies problematic because the researches would not be able to known the genetic contributions from the biological parents.