Final answer:
The first stage triggered upon encountering a stressor is the alarm stage of General Adaptation Syndrome, which involves the fight-or-flight response.
Step-by-step explanation:
The stage that is triggered when we first encounter a stressor is the alarm stage of General Adaptation Syndrome (GAS), as described by Hans Selye. This stage is characterized by the body’s immediate reaction to a threat, known as the fight-or-flight response, which is mediated by hormones like epinephrine and norepinephrine. For example, if someone wakes up to find their house on fire, their body will enter the alarm reaction, providing energy to manage the situation through various physiological changes.