What kind of perm uses heat to set the curls?

Heat-activated or thermal perm is the correct choice because it specifically utilizes heat to alter the hair's structure and create curls. This process involves the application of a chemical solution followed by the application of heat, typically through a perming machine or heat rods, which activates the chemical agents. The heat helps to break down the disulfide bonds in the hair, allowing it to reshape into the desired curl pattern more effectively, resulting in a longer-lasting and more defined curl.

In contrast, a cold perm does not use heat; it relies solely on chemical solutions to set the curls. Digital perms combine elements of both thermal and traditional perming techniques, but they utilize a different, often more controlled method involving digital temperature settings. Spiral perms focus on the method of wrapping the hair to produce a specific curl shape rather than the heat application process.