Which of the following driving practices is also illegal?

Listening to music through headphones that cover both ears is illegal while driving because it significantly impairs a driver's ability to hear important sounds from the environment, such as sirens from emergency vehicles, honking from other drivers, or warnings from pedestrians. This distraction can lead to dangerous situations since the driver may be unaware of critical auditory cues that are essential for safe driving.

Adjusting your outside mirrors is a necessary and often mandatory practice to ensure proper visibility while driving, and it can be done safely as long as it is performed before or after driving, not while the vehicle is moving. Transporting an unrestrained animal can pose risks to both the animal and driver, but it is not specifically illegal unless it creates a hazardous condition. Using a mobile phone while driving is also illegal in California unless it is hands-free, as it can lead to severe distractions and reduce the driver's attention on the road. However, the primary focus of the correct answer relates to how listening to music can compromise a driver’s awareness of surrounding traffic and sounds, making it particularly dangerous.