The structural and functional unit of nervous system is calledA neuron     B nerve C nephron     D  spinal cord

The correct answer is A. Neuron.

Step 1: Understand the question. The question is asking for the structural and functional unit of the nervous system.

Step 2: Analyze the options.

Option A: Neuron - This is a specialized cell that transmits nerve impulses. It is indeed a fundamental unit of the nervous system both structurally and functionally.

Option B: Nerve - This is a bundle of fibers that transmits impulses of sensation to the brain or spinal cord, and impulses from these to the muscles and organs. It's not the fundamental unit, but rather composed of many neurons.

Option C: Nephron - This is the fundamental unit of the kidney, not the nervous system.

Option D: Spinal cord - This is a major part of the central nervous system that conducts sensory and motor nerve impulses to and from the brain. It's not the fundamental unit, but a major component of the system.

Step 3: Choose the correct answer. Based on the analysis, the neuron (Option A) is the structural and functional unit of the nervous system.