Final answer:
The term referring to an individual's physiological reaction to a threat is the Fight or flight response, a crucial survival mechanism that is also triggered by modern psychological stressors.
Step-by-step explanation:
The term that refers to an individual's physiological response to a perceived threat is D) Fight or flight response. This response is an involuntary reaction mediated by the nervous and endocrine systems, which primes the body to either confront or flee from the threat. Characteristics of the fight or flight response include an increased heart rate, heightened senses, and a surge of adrenaline, which collectively prepare the body to handle the situation. While this response was crucial for our ancestors in surviving immediate physical dangers, it is also triggered in modern times by psychological threats, such as giving a presentation or taking a big test, despite no real physical danger being present.