Which tool is commonly used to roll the hair in a perm?

Perm rods are specifically designed tools used in the perming process to shape and secure the sections of hair being chemically treated. They come in various sizes and materials to create different curl patterns and sizes, allowing for customization according to the client's desired look. When the hair is wrapped around the perm rod, the rod holds the hair in place while the perm solution is applied, facilitating the transformation of straight hair into curls or waves during the chemical process.

The other tools mentioned serve different purposes in hair styling. Hair clips are typically used to section hair and keep it organized during various treatments, but they don't create curls or hold hair in a specific shape for a perm. Combs are essential for detangling and sectioning but are not used to roll hair. Straighteners, meanwhile, are used to smooth and straighten hair, which is contrary to the aim of perming. Therefore, perm rods are uniquely suited for the perming process, making them the correct choice for this question.