**Core Concept**
Kunkel's test is a laboratory technique used to detect the presence of a specific substance in the blood. This test is based on the principle of precipitation, which occurs when a antigen-antibody complex forms in the presence of a specific substance.

**Why the Correct Answer is Right**
Kunkel's test is specifically used to detect the presence of **Hemoglobin F (Fetal Hemoglobin)** in the blood. The test involves the addition of a reagent that contains anti-hemoglobin F antibodies, which bind to the hemoglobin F molecules in the blood sample, causing a visible precipitate to form. This precipitate indicates the presence of hemoglobin F in the blood. Hemoglobin F is a type of hemoglobin that is present in the blood of fetuses and is responsible for transporting oxygen from the mother's bloodstream to the fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Kunkel's test is not used to detect **Bilirubin**, which is a breakdown product of hemoglobin but is not directly related to the test.

**Option B:** This option is incorrect because Kunkel's test is not used to detect **Hemoglobin A1c (HbA1c)**, which is a form of hemoglobin that is used to monitor blood glucose levels in people with diabetes.

**Option D:** This option is incorrect because Kunkel's test is not used to detect **Lactate Dehydrogenase (LDH)**, which is an enzyme that is involved in the breakdown of lactate and pyruvate in the blood.

**Clinical Pearl / High-Yield Fact**
Kunkel's test is an important tool for diagnosing fetomaternal hemorrhage, which is a condition where fetal blood cells enter the maternal bloodstream. This can occur during pregnancy or childbirth and can lead to complications such as anemia or thrombocytopenia.

**Correct Answer:** C.