Hemolytic disease of the newborn is most likely to occur when the mother is:

Hemolytic disease of the newborn primarily occurs in situations of Rh incompatibility between the mother and fetus. This condition arises when an Rh-negative mother carries an Rh-positive fetus, due to the potential for the mother's immune system to recognize the Rh-positive blood cells as foreign.

When the mother is Rh-negative and the fetus is Rh-positive, the mother's body may produce antibodies against the Rh factor. If these antibodies cross the placenta, they can attack and destroy the red blood cells of the fetus, leading to hemolysis. This process results in hemolytic disease, which can cause complications such as anemia, jaundice, and even heart failure in the fetus or newborn.

In contrast, when the mother is Rh-positive, she typically does not have antibodies against the Rh factor and thus is less likely to experience this immune response. Similarly, blood group incompatibilities (as indicated in the other options regarding blood types A, B, and O) do not typically lead to hemolytic disease in the same way that Rh incompatibility does. Blood type incompatibilities can cause various immune reactions but are not as severe or common as Rh incompatibility leading to hemolytic disease of the newborn.