An auto accident rendered chris's nervous system unable to send messages for him to breathe, so he is on a respirator. which brain structure was most likely damaged in the accident?

Medulla

Explanation:

The medulla oblongata in our brain serves as a primary control of our respiration system, since it is the one that send the signals that control the movement of the muscles that involved in the breathing process. It is basically a mediator between your brain and your lungs.

If medulla is damaged, there will be nothing that conveys the orders from the brain to make the muscle movement necessary for your respiration. This is what makes chris unable to breath without the respirator to replace the function of his lung.