Final answer:
Norepinephrine is the neurotransmitter and hormone involved in the body's stress response, and its primary functions include increasing alertness, arousal, and reaction time during stressful situations.
Step-by-step explanation:
The neurotransmitter and hormone involved in stress response, alertness, arousal, emotion regulation, and attention with the main function of increasing alertness and arousal as well as speeding up reaction time is norepinephrine (D). Norepinephrine, also known as noradrenaline, is a catecholamine hormone and neurotransmitter that plays a key role in the body's fight-or-flight response during stressful situations. It is released by the adrenal medulla and certain neurons and is responsible for a range of physiological changes such as increasing blood glucose levels, oxygen availability, heart rate, and blood pressure while constricting blood flow to less essential organs.