Answer:
The reticular activating system (RAS)
Step-by-step explanation:
The reticular activating system is a part of the brain that can be found just above where the spinal cord is connected to the brain. It is projected anteriorly to the hypothalamus to control behavior and on the posterior of the thalamus to activate awake signals.
The RAS is in charge of attention and sleep-wake cycles in humans, therefore the part of Alex's brain that was most likely activated by the sudden explosion is The reticular activating system (RAS)