The answer is "Indirect observation".
In social research, there are two types of methods used; direct and indirect observation. In this case, since the population is not aware of the research it is clearly indirect observation. In this technique, it observed the effects of results compare to a direct observation which observes as it occurs.
The correct answer for the above question is D. that the peripheral system includes all the nerves and neurons outside the brain and the spinal cord. The nervous system comprises the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, consisting of the cranial, spinal, and peripheral nerves, together with their motor and sensory endings.
An example would be the coral and the clownfish