(A) Nephron
The nephron is the basic filtration unit of the kidney. Each kidney contains large numbers of nephrons packed together that filter nitrogenous wastes from the blood.
The passage clearly states: "Each kidney has large numbers of these filtration units called nephrons packed close together." Urethra carries urine out of the body; urinary bladder stores urine; neuron is a nerve cell — all distractors unrelated to filtration.