Which of the following is a common polishing agent used in coronal polishing?

A common polishing agent used in coronal polishing is calcium carbonate. This substance is favored for its gentle abrasive properties, making it effective in removing plaque and stains from the tooth surface without causing damage to the enamel. Calcium carbonate is typically found in various prophy pastes, which are specifically formulated for dental cleaning and polishing procedures.

Other substances listed, like fluoride varnish and stannous fluoride, are primarily used for the prevention of dental caries and are not abrasive polishing agents. Fluoride varnish helps strengthen tooth enamel and provides a protective barrier against decay, while stannous fluoride has antimicrobial properties and assists in remineralization but is not intended for polishing teeth. Chlorhexidine gluconate is an antiseptic used primarily for its antibacterial properties in managing plaque and gingivitis, making it beneficial for oral hygiene but unsuitable as a polishing material. Thus, among the options, calcium carbonate is recognized for its role in mechanical polishing during coronal polishing procedures.