Final answer:
When Joe hears footsteps and feels afraid, his amygdala is activated, signaling the hypothalamus to initiate a fight-or-flight response, not the hippocampus or cerebellum.
Step-by-step explanation:
As Joe walks to his car late at night and hears footsteps behind him, feeling afraid and gripping his keys to quicken his pace, it is likely that Joe's amygdala has been activated. The amygdala is responsible for regulating emotions, such as fear and aggression. When Joe experiences fear, his amygdala sends signals to the hypothalamus triggering the sympathetic fight-or-flight response. Furthermore, the hypothalamus stimulates the release of stress hormones which are part of the body's response to fear or stressful situations.
The hippocampus is associated with memory and would not be responsible for Joe's immediate fear response. The cerebellum is involved in motor control and procedural memories and is not typically associated with emotional responses like fear. Hence, the correct answer to what has been activated in Joe's brain is option C) amygdala.