Which of the following actions is illegal while driving?

Using a cellphone while driving is illegal because it greatly increases the risk of accidents. Distracted driving, especially due to mobile device use, diverts a driver's attention from the road, impacting their reaction time and awareness of surroundings. Many states, including Montana, have implemented laws that restrict or prohibit the use of handheld devices while driving to enhance road safety.

The other actions listed, such as adjusting the radio while stopped, typically do not pose the same level of risk as they occur when the vehicle is not in motion. Eating while driving can be distracting but is not universally illegal, though it is often discouraged. Using your mirrors is a necessary action for safe driving, as it allows you to be aware of your surroundings and make informed driving decisions.