Nephron is the structural and functional unit of the kidney (excretory system) that filters blood. Neuron is the structural and functional unit of the nervous system that transmits electrical impulses.
This is a 1-mark difference question, so one clear contrasting point is sufficient. The examiner expects you to identify what each unit belongs to (kidney vs. nervous system) and its basic function (filtration/excretion vs. impulse transmission). Avoid writing lengthy descriptions.