**Core Concept**
Erythroblastosis fetalis is a condition where the immune system of the mother reacts to the Rh-positive blood of the fetus, leading to the production of antibodies that can cross the placenta and destroy fetal red blood cells. This condition is most commonly seen in Rh-negative mothers carrying Rh-positive fetuses.

**Why the Correct Answer is Right**
The risk of erythroblastosis fetalis is highest when the mother is Rh-negative and the father is Rh-positive, as this can result in the mother becoming sensitized to Rh-positive blood. This sensitization occurs when the mother's immune system is exposed to Rh-positive blood during delivery or abortion of an Rh-positive fetus. The mother's body produces antibodies against Rh-positive blood, which can then cross the placenta in future pregnancies and attack the red blood cells of an Rh-positive fetus, leading to hemolysis and potentially severe anemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified, but typically, the combination of Rh-negative mother and Rh-negative father would not result in the mother becoming sensitized to Rh-positive blood. However, if the mother is already sensitized, this combination would not increase the risk.
* **Option B:** This option is not specified, but typically, the combination of Rh-positive mother and Rh-positive father would not increase the risk of erythroblastosis fetalis, as the mother's immune system would not be sensitized to Rh-positive blood.
* **Option C:** This option is not specified, but typically, the combination of AB-negative mother and B-positive father would not increase the risk of erythroblastosis fetalis, as the mother's immune system would not be sensitized to the father's blood type.

**Clinical Pearl / High-Yield Fact**
To prevent erythroblastosis fetalis, Rh-negative mothers should receive Rh immunoglobulin (RhIG) during pregnancy and after delivery if the baby is Rh-positive. This can prevent the mother's immune system from becoming sensitized to Rh-positive blood.

**Correct Answer: C.**