Para-phenylene-diamine is an ingredient found in which type of product?

Para-phenylene-diamine (PPD) is a chemical compound commonly used in the formulation of permanent oxidative hair colors. This ingredient plays a crucial role in the hair dyeing process as it helps to create a lasting change in the hair's color by penetrating the hair shaft and reacting with other components in the dye to develop the desired shade.

In contrast, the other products listed do not typically contain PPD. Shampoos and conditioners are generally formulated to cleanse and condition the hair without the need for strong colorants, whereas hair gels are primarily used for styling purposes and do not include permanent dyeing agents like PPD. This specificity underscores the role of PPD in achieving semi-permanent or permanent hair color, which is why the correct choice pertains to permanent oxidative colors.