Which part of the nervous system will be most active when martin, a backpacker, acts quickly in response to a wild animal approaching him ?

Autonomic nervous system; Sympathetic nervous system

Explanation:

There are two kinds of nervous systems, central nervous system, and peripheral nervous system. Within the peripheral nervous system, there is the somatic nervous system and the autonomic nervous system. Within the autonomic nervous system there is the sympathetic nervous system and the parasympathetic nervous system.

The sympathetic nervous system controls the release of Adrenalin from the renal glands which affects the heart rate of the animal/human.

one would be wind another would be water density