Which stain would commonly be used to visualize the flagella of bacteria?

The correct choice for visualizing the flagella of bacteria is a structural stain. Structural stains are specifically designed to highlight particular cellular components that may not be well visualized using standard staining techniques. The flagella, being thin and delicate, require a specialized approach to ensure they are adequately visualized under a microscope.

Structural stains often involve methods like the use of mordants or specific chemicals that can bind to the flagella, enhancing their visibility against the cellular background. This is crucial because flagella are typically much narrower than other cellular structures, making them challenging to observe with regular stains.

While the Gram stain focuses on the cell wall characteristics of bacteria and the negative stain provides contrast by staining the background rather than the cells themselves, these methods do not specifically target structures like flagella. Simple stains, while helpful for overall cell visualization, lack the specificity required for detailed observation of such minute features. Hence, using a structural stain is the most effective way to visualize flagella and understand their role in bacterial motility.