Which of the following is a requirement for a cosmetic to be considered safe for use?

A cosmetic product is deemed safe for use primarily because it cannot contain any harmful substances. This requirement is crucial to ensure the health and safety of consumers. The presence of harmful substances in cosmetics can lead to adverse reactions, skin irritation, or long-term health effects. Regulatory bodies, such as the FDA in the United States, set strict guidelines and safety standards for cosmetic formulations to ensure they are free from harmful ingredients. Therefore, the focus on harmful substances directly correlates to consumer safety, making this criterion essential for a cosmetic's safety profile.

The other options do not directly relate to the safety of a cosmetic product. For example, price does not indicate safety; rather, it pertains to market competition. Packaging aesthetics or celebrity endorsements may attract customers but have no bearing on whether a product is safe for use.