**Core Concept**
The Metabisulfite test is a laboratory method used to detect the presence of abnormal hemoglobin, specifically hemoglobin S (HbS), which is associated with sickle cell disease. This test takes advantage of the characteristic properties of HbS to identify individuals with sickle cell trait or disease.

**Why the Correct Answer is Right**
The Metabisulfite test works by reducing the oxygen affinity of normal hemoglobin (HbA), causing it to release oxygen and bind to the sulfite. However, HbS has a higher affinity for oxygen than HbA, and when exposed to sulfite, it undergoes a conformational change that leads to its precipitation. This precipitation is indicative of the presence of HbS. The test is particularly useful for detecting carriers of sickle cell disease, as they may not exhibit clinical symptoms but can still be identified through this laboratory test.

**Why Each Wrong Option is Incorrect**
**Option A:** The Coombs' test is used to detect the presence of antibodies or complement proteins that are attached to the surface of red blood cells, typically in cases of autoimmune hemolytic anemia.

**Option B:** The Osmotic fragility test is used to assess the integrity of red blood cells by measuring the amount of hemolysis that occurs when cells are exposed to varying concentrations of saline solution. This test is more relevant to diagnosing conditions like hereditary spherocytosis.

**Option D:** The Sucrose hemolysis test is used to diagnose glucose-6-phosphate dehydrogenase (G6PD) deficiency, a condition characterized by the breakdown of red blood cells in response to oxidative stress.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Metabisulfite test is a screening tool and may not be 100% sensitive or specific for detecting HbS. Further testing, such as hemoglobin electrophoresis, may be necessary to confirm the diagnosis.

**✓ Correct Answer: C. Metabisulfite test**