Which of the following is a common styling tool used to achieve curls?

A hair roller is a common styling tool used to achieve curls because it works by wrapping sections of hair around a cylindrical shape, allowing the hair to set in a curled position as it dries or cools. This method can create a variety of curl sizes, depending on the diameter of the roller used. Using hair rollers allows for curls that can be soft and bouncy or tighter, depending on how the hair is wrapped and the specific roller type.

In contrast, a flat iron typically straightens hair by clamping and pressing it between heated plates; it may be used to create curls, but that is not its primary purpose. A comb is primarily a smoothing and detangling tool and does not create curls. Similarly, a wide-tooth brush is used for detangling and styling but is not designed for curling hair. Each of these other tools serves different styling purposes, while the hair roller is specifically designed to create curls effectively.