**Core Concept**
The question is testing the knowledge of a laboratory test used to detect maternal sensitization, which is a crucial aspect of prenatal care and management of RhD incompatibility. Maternal sensitization occurs when an RhD-negative mother is exposed to RhD-positive fetal red blood cells, leading to the production of antibodies against RhD.

**Why the Correct Answer is Right**
The correct answer is **Kleihauer-Betke test**. This test is used to detect fetal-maternal hemorrhage (FMH) and maternal sensitization by demonstrating the presence of RhD-positive fetal red blood cells in the maternal circulation. The test involves treating a blood smear with acid elution, which removes adult hemoglobin, leaving the fetal hemoglobin intact. RhD-positive fetal red blood cells are then visualized under a microscope.

**Why Each Wrong Option is Incorrect**
**Option A:** The Coombs test is used to detect the presence of antibodies against red blood cells, but it is not specific for detecting maternal sensitization or fetal-maternal hemorrhage.

**Option B:** The Apt test is used to differentiate between fetal and adult hemoglobin, but it is not a test for maternal sensitization.

**Option C:** The Duffy test is not a relevant test for maternal sensitization or fetal-maternal hemorrhage.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the Kleihauer-Betke test is not a quantitative test, and it only detects the presence of RhD-positive fetal red blood cells in the maternal circulation.

**Correct Answer:** B. Apt test