**Core Concept**
The Apt test is a chemical test used to differentiate between fetal and adult blood, based on the presence of fetal hemoglobin and its characteristic properties.

**Why the Correct Answer is Right**
The Apt test takes advantage of the fact that fetal hemoglobin is more resistant to acid denaturation than adult hemoglobin. When a sample of blood is mixed with a solution of sodium hydroxide (NaOH), fetal hemoglobin will retain its globin chains, whereas adult hemoglobin will be denatured and release its globin chains. The presence of globin chains in the supernatant can be detected using a test strip or a chemical reagent, indicating the presence of fetal blood. This test is particularly useful in cases of maternal hemorrhage or suspected fetal-maternal transfusion.

**Why Each Wrong Option is Incorrect**
**Option A:** The Kleihauer-Betke test is used to detect fetal-maternal hemorrhage by staining fetal hemoglobin in maternal blood, but it is not used to differentiate between fetal and adult blood.

**Option C:** The Bubbling test, also known as the "nitric acid test," is used to detect the presence of fetal blood by observing the formation of bubbles when the blood is mixed with nitric acid, but it is not a reliable method and can be contaminated by other substances.

**Option D:** The Osmotic fragility test is used to diagnose hereditary spherocytosis, a condition characterized by red blood cells that are more prone to hemolysis under hypotonic conditions, but it is not used to differentiate between fetal and adult blood.

**Clinical Pearl / High-Yield Fact**
The Apt test is a simple and reliable method for differentiating between fetal and adult blood, but it should not be used as the sole criterion for diagnosing fetal-maternal hemorrhage, as it may not detect small amounts of fetal blood.

**✓ Correct Answer: B. Apt test**