**Core Concept**
The best test for monitoring sensitized Rh negative mothers is one that detects the presence of anti-Rh antibodies, which can cause hemolysis in the fetus. This is a critical concern in obstetrics as Rh incompatibility can lead to severe anemia and even death in the fetus.

**Why the Correct Answer is Right**
The best test for monitoring sensitized Rh negative mothers is the **Kleihauer-Betke test**. This test detects the presence of fetal hemoglobin (HbF) in the maternal circulation, which indicates that the mother has anti-Rh antibodies. The Kleihauer-Betke test works by using a pH-dependent dye that stains HbF in a red color, while adult hemoglobin (HbA) remains unstained. This test is particularly useful in monitoring the severity of hemolysis and guiding the need for fetal transfusions.

**Why Each Wrong Option is Incorrect**
**Option A:** The **Direct Coombs test** detects the presence of antibodies on the surface of red blood cells, but it does not specifically detect fetal hemoglobin and is not the best test for monitoring sensitized Rh negative mothers.

**Option B:** The **Indirect Coombs test** detects the presence of antibodies in the maternal serum, but it does not provide information on the severity of hemolysis or guide the need for fetal transfusions.

**Option C:** The **Rh antibody titer** measures the level of anti-Rh antibodies in the maternal serum, but it does not provide information on the presence of fetal hemoglobin and is not the best test for monitoring sensitized Rh negative mothers.

**Clinical Pearl / High-Yield Fact**
The Kleihauer-Betke test is a critical tool in obstetrics for monitoring sensitized Rh negative mothers, and it should be performed regularly in these patients to detect the presence of anti-Rh antibodies and guide the need for fetal transfusions.

**Correct Answer:** C. Kleihauer-Betke test.