Final answer:
The India ink stain is the most commonly used stain for visualizing the capsule of Cryptococcus, as it stains the background but not the capsule, resulting in a clear halo around the cells.
Step-by-step explanation:
The most commonly used stain to visualize the capsule of Cryptococcus is c) India ink stain. This staining technique is particularly useful for identifying encapsulated yeasts and bacteria such as Cryptococcus neoformans. In this technique, the India ink stains the background but not the capsule, which results in a clear halo surrounding each cell, making the capsules visible as a negative image. Unlike other stains that color the cells directly, the India ink or similar dyes used in negative staining methods do not penetrate the capsules. This creates a contrast against the stained background and reveals the presence of the capsule without the need to heat-fix the specimen, reducing the risk of distorting the cells' shape.
Negative staining is a valuable diagnostic tool in microbiology as it aids in the identification of certain bacteria and yeasts based on the presence of a protective capsule, which is a factor that can be related to the microbe's virulence. Although other staining methods exist, such as Gram stain, Acid-fast stain, and Wright stain, they serve different purposes and are not specifically used for capsule visualization.