Answer:
The correct answer is letter "D": nucleus.
Explanation:
In neurology, the nucleus is the part of the neuron that contains the genetic material in the form of chromosomes. Neurons need to produce a great number of proteins and most of the neuronal proteins are synthesized in the nucleus. The nucleus is usually in the center of a neuron and is the most visible part of it.
Therefore, in the metaphor of the example the computer system that stores the information of prospective clients will act as the nucleus of a neuron.