**Core Concept**
Reticulocytes are immature red blood cells that are present in the peripheral blood for a short period of time. They contain residual RNA and a characteristic network of RNA-containing fibrils, which can be stained using specific dyes.

**Why the Correct Answer is Right**
Brilliant cresyl blue is a supravital stain that selectively stains the RNA-containing fibrils in reticulocytes, allowing them to be visualized under a microscope. The dye binds to the RNA, highlighting the characteristic reticular pattern. This staining technique is a valuable tool in hematology for identifying reticulocytes and assessing erythropoiesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Methylviolet is a basic dye used for staining many types of cells, but it does not selectively stain reticulocytes. It can stain the cytoplasm of many cell types, making it unsuitable for reticulocyte identification.
**Option C:** Sudan black is a lipophilic dye used for staining lipids and lipoproteins, but it is not specific for reticulocytes. It can stain other types of cells and cellular components, but it does not have the specificity required for reticulocyte identification.
**Option D:** Indigo carmine is a basic dye used for staining many types of cells, but it does not selectively stain reticulocytes. It can stain the cytoplasm of many cell types, making it unsuitable for reticulocyte identification.

**Clinical Pearl / High-Yield Fact**
Reticulocytes can also be stained using new methylene blue, but brilliant cresyl blue is a more specific and commonly used stain for this purpose. It is essential to use a supravital stain like brilliant cresyl blue to avoid damaging the reticulocytes during the staining process.

**✓ Correct Answer: B. Brilliant cresyl blue**