Hiking along a remote trail, Chen surprises a mother bear with her cubs. Chen's pupils dilate, his breathing rate increases, and his heart rate soars. Which part of the nervous system is MOST involved in this process?

the sympathetic division of autonomic nervous system.

Explanation:

When the person experiences any stress or emergency conditions, the sympathetic division is activated. It triggers the release of stress hormones from the adrenal medulla. There is a series of physiological events called flight or fight response. It includes an increased breathing rate and increased heart rate to supply the extra oxygen as required under these conditions. The blood vessels supplying the blood to skeletal muscles are dilated and the pupils are dilated. All these responses prepare the person for emergency conditions such as facing a wild animal.