What is a common sign that a hair product is not suitable for a client?

A common sign that a hair product is not suitable for a client is when the client experiences an adverse reaction such as irritation or itching. This reaction indicates that the product may contain ingredients that are not compatible with the client's skin or hair type, or that the client may have an allergy to one or more components of the product. It is crucial for the health and comfort of the client to discontinue use of any product that causes these negative reactions, and to seek alternatives that are gentler and more appropriate for their specific needs.

The other options might suggest preferences or characteristics that could vary from person to person, but they do not directly indicate incompatibility or a health concern with the product itself. For example, shiny hair or a specific scent does not necessarily correlate with a product's suitability; it could simply reflect the formulation of the product. Hair taking longer to style may relate to the product’s effectiveness but does not indicate an adverse physical reaction. Therefore, the presence of irritation or itching is a more definitive sign that a product should not be used.