Final answer:
Epinephrine is the factor that decreases airway resistance by causing the dilation of the airways, which is part of the body's “fight or flight” response.
Step-by-step explanation:
The factor that decreases airway resistance is epinephrine. This is because epinephrine is involved in the body's “fight or flight” response and when released, it leads to the dilation of the airways, thereby reducing resistance. In contrast, activation of the parasympathetic nervous system typically results in bronchoconstriction, which increases airway resistance. Histamine, released from mast cells during an allergic reaction, can also cause bronchoconstriction and increase airway resistance. Therefore, out of the three options given - activation of the parasympathetic nervous system, epinephrine, and histamine - epinephrine is the factor that decreases airway resistance.