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

In the context of eyelash extension services, substances that are safe and permissible to use are crucial for the well-being of both the technician and the client. Caffeine may be incorporated in beauty products due to its potential benefits like reducing puffiness or stimulating blood flow. While its use in aesthetic treatments is less common compared to topical applications, if caffeine is included in a product being used, it is generally regarded as safe and non-toxic in the concentrations typically found in cosmetic formulations.

Conversely, alcohol, formaldehyde, and acetone have more significant implications for safety and health. Alcohol can be too drying and irritating for the skin, and might not be suitable in certain applications as it can compromise the integrity of the adhesives used in eyelash extensions. Formaldehyde, though sometimes found in products like eyelash adhesives, has known health risks and can provoke allergic reactions or irritations. Acetone is a potent solvent often used to remove artificial nails or nail polish, and it can be excessively drying, potentially harming the skin around the eyes.

Thus, while caffeine can be safely used in certain contexts, the other substances pose risks that are better avoided in the delicate application process involved in eyelash extensions. Understanding the properties and safety profiles of these substances