While Hiking with a Friend in the Mojave Desert, Lonny

While hiking with a friend in the Mojave Desert, Lonny comes face-to-face with a rattlesnake on the trail. Lonny rapidly enters the first stage of Selye's General Adaptation Syndrome. His brainstem initiates the release of the neurotransmitter norepinephrine, which increases his vigilance and fear. This is part of Lonny's

A) cognitive response.
B) behavioral response.
C) emotional response.
D) physical response.