Answer:
anxiety
Step-by-step explanation:
Anxiety may be defined as a disorder of one's mental health which is characterized by the feelings of anxiety, worry, or fear which are so strong that it can interfere one's daily activities and behaviors.
Anxiety is our body's natural response to stress.
In the context, whenever Bill is in the aeroplane, he feels fear of flying and his nervous system automatically arouses because of the fear. This behavior is called as anxiety.