Which of the following formulations is appropriate for a topical anesthetic?

A liquid formulation is appropriate for a topical anesthetic because it allows for easy application to the skin or mucous membranes where localized sensation is to be blocked. Liquid formulations can provide a uniform distribution over the area being treated, which is crucial for effectiveness in numbing sensation. They can also facilitate the absorption of the anesthetic into the tissues, enhancing its performance.

Topical anesthetics are designed to act directly on the nerve endings in the skin or mucous membranes, and a liquid can penetrate these tissues effectively. Furthermore, liquids often allow for specific dosing and control over the amount applied, optimizing the anesthetic effect.

Other formulations like foam, shampoo, and granules may not provide the necessary characteristics for effective topical anesthesia. Foams might not deliver the anesthetic uniformly or effectively penetrate the skin. Shampoos might not be suitable due to their intended use on the scalp rather than localized pain relief, and granules typically require dissolution or application to be effective, which complicates their use in achieving immediate localized anesthesia.