Which of the following is considered a levigating agent used in compounding?

Glycerin is recognized as a levigating agent in compounding due to its ability to help reduce the particle size of powders when mixed with a solid. Levigation is the process of grinding a powder with a liquid to create a smoother, more uniform mixture, facilitating better incorporation into formulations, enhancing the product's texture, and improving overall stability.

Glycerin, being a viscous liquid, allows for a better wetting of the solid material, thus making it easier to mix and achieve the desired consistency for compounded formulations. Its hydrophilic properties also make it compatible with many powders, further serving the purpose of a levigating agent effectively.

In contrast, the other substances listed do not possess the primary qualities necessary to function as levigating agents. Sorbitol, while it has some lubricating and sweetening properties, is not typically used for the purpose of particle size reduction. Calcium phosphate is primarily used as a filler or thickening agent and lacks the properties needed for levigation. Starch can serve as a binder or thickening agent, but it does not facilitate the levigation process effectively like glycerin does.