While hiking in the mountains in Alaska, Pete encountered a giant grizzly bear. He immediately felt his heart rate increase, his breathing become rapid, and sweat drip down his back. These events were most likely due to effects of the neurotransmitter called .

epinephrine

Explanation:

Epinephrine is an example of neurotransmitter in the body , which is always being released into the bloodstream  when there is fear or anger. It is known to be secreted from the medulla of one of the endocrine gland, adrenal gland.

Its secretion has some effects like an increase in heart rate, muscle strength,  and sugar metabolism.

In this case, when Pete encountered the giant bear and felt his rate to be increased, it showing the effect of the epinephrine that has been secreted into the bloodstream.

d ) power this is the right answer

which ever one is showing doing the same exercise in time intervals such as 30 seconds or a minute several times.