**Core Concept**
Reticulocytes are immature red blood cells that are stained with specific dyes to quantify their count in peripheral blood smears. This test is crucial for assessing bone marrow function, particularly in cases of anemia or hemolysis.

**Why the Correct Answer is Right**
The correct answer is New Methylene Blue (NMB). New Methylene Blue is a supravital dye that stains the ribosomal RNA in reticulocytes, making them visible under a microscope. This dye binds to the RNA, causing the reticulocytes to appear as a network of dots or a reticulum. The count of reticulocytes is then compared to the total red blood cell count to determine the reticulocyte production index (RPI), which is a measure of bone marrow function.

**Why Each Wrong Option is Incorrect**
**Option A:** Brilliant Cresyl Blue is a supravital dye used to stain mast cells, not reticulocytes.
**Option B:** New Thioguanine is not commonly used for staining reticulocytes.
**Option C:** No specific information is available on a commonly used dye for staining reticulocytes by this name.

**Clinical Pearl / High-Yield Fact**
When interpreting reticulocyte counts, it is essential to consider the clinical context, including the patient's hemoglobin level, mean corpuscular volume (MCV), and presence of any underlying conditions that may affect bone marrow function.

**Correct Answer:** C. New Methylene Blue.