Can I apply for my biological parents if I am adopted?

Generally, when a child is formally adopted, i.e. through a formal adoption process with an Adoption Decree issued by an appropriate court, in the eyes of the law the relationship with the biological parents are severed and the relationship with the adoptive parents are recognized. If a child is not formally adopted, i.e. there is no court order and the family members merely treat the relationships as if the child were the child of different parents, then in the eyes of the law the relationship with the biological parents remains (and when the child becomes an adult, if he/she is a U.S. Citizen, the child may sponsor the parents to get immigration benefits in the Immediate Relative visa category).

It depends. If adoption in the US helped you to get US immigration status, then the answer is no.