Which of the following substances is a licensee allowed to use while performing a service?

The selection of caffeine as a substance that a licensee is allowed to use while performing a service is based on its nature and safety profile. Caffeine is generally recognized as a safe ingredient and is often utilized in cosmetic formulations for its stimulating properties. For example, it may be included in products aimed at improving circulation or reducing puffiness.

In contrast, the other substances mentioned—such as alcohol, formaldehyde, and acetone—raise concerns regarding their potential side effects and safety when used in beauty treatments. Alcohol can be overly drying and may irritate the skin. Formaldehyde is a known irritant and has been linked to health risks, making its use in cosmetic applications highly regulated and often prohibited. Acetone, while effective as a solvent, can be harsh on the skin and should be used cautiously, if at all.

Thus, caffeine stands out as a more beneficial and safer choice, aligning with best practices in the cosmetics and beauty industry where the focus is on health and safety during service performance.