True answer:
To live with his/her natural parents
By law, adopted child has all the legal rights of the member of the family that raises him or her. A foster child does not.
On the contrary, many adopted people worked to change the laws. They argue that people have a right to know their identity. They said that keeping secret their adoptions records violate their right.
On the other hand, many people feel adoption records should be keep secret. If not, natural parent might change their mind years later and take the child away from the parents who raised him or her.