Which structure is at work when the body deals with reflex actions?

The correct answer is spinal cord.

Explanation:

A reflex action is an instantaneous response movement caused due to stimulus. This is an involuntary action which is controlled by the neurons of the spinal cord through a pathway called reflex arc.  

This reflex arc starts when excited receptors send signals in the form of impulse through sensory neurons. This impulse travels to the dorsal roots of the spinal cord via afferent neuron or sensory neuron where interneuron or association neuron send the signals via motor neurons to muscle cells which respond.

Thus, the spinal cord is the correct answer.