Which of the following is not considered a common basic stain?

D. Eosin is not considered a common basic stain because it is an acidic dye. Basic stains work by binding to negatively charged components within cells, such as nucleic acids and proteins. Eosin, having an anionic nature, is used primarily in histology and cytology to stain cytoplasmic structures and is often employed in the context of tissue samples to provide contrast against other stains, but it's not used as a basic stain in microbiological contexts.

In contrast, methylene blue, crystal violet, and safranin are all basic stains. Methylene blue is often used to visualize cell morphology because it penetrates the cytoplasm and stains the nuclei of the cells. Crystal violet is commonly used in the Gram staining procedure to differentiate between Gram-positive and Gram-negative bacteria. Safranin is frequently the counterstain in the Gram staining process, providing visual contrast to crystal violet. These basic stains share the property of interacting positively with negatively charged components of cells, thus enriching their utility in microbiology lab practices.