The intelligence scores of adopted children are least likely to correlate positively with the intelligence scores of their:

The correct answer to this open question is the following.

Your question is incomplete. You missed the end of the question that is “…their adopted siblings during:”

Although you did not attach options for this question, we can say the following.  

The intelligence scores of adopted children are least likely to correlate positively with the intelligence scores of their early adulthood.

Scientists and sociologists know that adopted children usually have different personalities than their adoptive parents for obvious reasons. That is why during infancy, parents can exert a positive influence on children by exemplifying the kind of behaviors they want the children to have in social life. This aspect can determine the way adoptive children can develop or learn that set of new skills or personalities taught by the parents.