Alicia climbs into the passenger side of her boyfriend Bo’s car. In an attempt to impress Alicia, Bo speeds away from the curb as Alicia’s head presses against the headrest. What allows Alicia to sense this linear acceleration?

Hair cells in the vestibular system bend with the forward motion.

Explanation:

The main function of the ear is the enable to listen the different frequency of the sounds and the maintenance of the equilibrium of the body. The three main divisions of human ear - outer ear, middle ear and inner ear.

The vestibule system plays an important role in the equilibrium maintenance. The hair cells has the ability to sense the equilibrium and respond against the change. These cells move against the motion or the gravity depends on the movement. As the information given in the question, hair cells in the vestibular system bend with the forward motion helps in equilibrium maintenance.

the answer is:

carbon dioxide