Which of the following materials is used to polish acrylic denture bases?

Flour of pumice is the appropriate material used to polish acrylic denture bases because it is a fine abrasive that effectively smoothens and enhances the finish of acrylic surfaces. This particular polishing agent is non-toxic, readily available, and is used widely in dental practices for its ability to produce a high shine on acrylic resin without scratching it.

The characteristic of flour of pumice that makes it suitable for polishing acrylic denture bases is its fine particle size, which provides a gentle abrasive action. This helps in creating a shiny and smooth surface, which is essential for both esthetic purposes and the comfort of the patient, as rough surfaces can lead to irritation in the oral cavity.

The other materials mentioned, while they may have their uses in various applications, are not commonly recommended for polishing acrylic denture bases. For example, sand is coarser and can cause significant scratching on the acrylic surface, while silex, often used in other contexts, does not provide the same finesse required for dental applications. Chalk can also be too abrasive for delicate materials like acrylic, and typically used in different polishing scenarios. Thus, flour of pumice is specifically chosen for its properties that align with the needs of polishing denture bases effectively.