Correct Answer: Neurons

Solution : Given:

Kidneys : Nephron :: Central Nervous System : ?

The relationship is that the nephron is a structural and functional unit within the kidneys . Nephrons are found within the kidneys and play a crucial role in their functioning.

Now, let's apply the same logic to the second part of the analogy:

Central Nervous System : Neuron

The relationship here is that a neuron is a structural and functional unit within the central nervous system (CNS) . Neurons are the basic building blocks of the CNS and play a crucial role in transmitting signals and information within the nervous system, just as nephrons do within the kidneys.

So, the related word for "Central Nervous System" is "Neuron" .

Hence, option fourth is correct.