What is the granular medium and coating on greensand?

The correct answer is based on the components and properties of greensand, which is commonly used in water treatment processes. Greensand is primarily composed of glauconite, a mineral that has the ability to remove iron, manganese, and hydrogen sulfide from water. In addition to glauconite, the medium is typically coated with manganese dioxide. This coating enhances the greensand's filtration capabilities, allowing it to oxidize iron and manganese in water, thereby improving the effectiveness of water treatment systems.

Glauconite's natural occurrence in a sand-like form, combined with its coating of manganese dioxide, is what makes greensand highly efficient for use in water filtration. This specific combination is essential for providing the chemical reactions needed during the filtration process to remove contaminants efficiently.

In contrast, the other options include a mixture of materials that do not reflect the true composition and functionality of greensand. For instance, silica sand and granite sand do not have the chemical properties for effective iron and manganese removal that glauconite does. Similarly, coatings such as activated carbon, calcium carbonate, and iron oxide serve different purposes that do not align with the role of greensand in filtration systems.