Which stain is reddish-brown and forms rapidly on exposed root surfaces and cementum?

The correct answer discusses the properties of stannous stain, which is known for its reddish-brown color that develops quickly on exposed root surfaces and cementum. Stannous fluoride, when used in dental care, can lead to a stain that appears reddish-brown due to the interaction with tooth structure, especially in areas of exposed dentin and cementum, where the fluoride can react with the proteins in the oral environment.

Understanding the context of staining in dentistry is important as various agents can produce differing results when applied to dental surfaces. Stannous stain commonly occurs due to its inherent properties in reaction to the oral environment, making it a significant consideration for dental professionals when assessing and managing tooth discoloration due to topical agents.