Janine completed several tours of duty in afghanistan. she suffers from ptsd. now, back home in texas, she is frightened by firecrackers and cars backfiring. the fact that these sounds scare her reflects a process of stimulus

It reflects a process of Stimulus Generalization
Stimulus Generalization refers to a situation when conditioned stimulus  has the tendency to evoke similar responses after it has been conditioned.
Due to The Fact janine has condiiton every sound as a source of danger when she's on the war zone, the similar sound may caused her to feel that she's still there even though she's came back home.