The filtration units of kidneys are called nephrons.
Explanation: A nephron is the functional unit of the kidney, responsible for filtering waste products from the blood and forming urine.
Key points about the other options:
Ureter: A tube that carries urine from the kidney to the bladder.
Urethra: A tube that carries urine from the bladder out of the body.
Neurons: Nerve cells that transmit electrical signals throughout the body.