The couple`s blood groups are AB positive and O negative, which means they can only pass on A, B, or O blood groups. They can`t pass on AB as one parent has the O blood group. Furthermore, positive Rh factor is dominant over negative, so it`s possible for them to have a child with any Rh type. Given that the three sons have blood groups A positive, B positive, and O positive, we can rule out A positive and B positive as the potential blood group of the adopted son for these blood groups are of their biological children. So, the adopted son`s blood group can either be O positive (Option 1) or something else not listed (Option 4). As we don`t have any data indicating any other blood groups, the most plausible answer is Option 1- O positive. But remember, `cannot be determined on the basis of the given data` is also a potential choice as we don`t inherently know the blood group of an adopted child based on the parents` blood groups.