Explanation:

- The correct answer is Nephron (Option 4). The nephron is the structural and functional unit of the kidney, responsible for filtering blood, reabsorbing needed substances, and excreting waste as urine.

- Medulla (Option 1) is a part of the kidney that contains the nephrons' loop of Henle and collecting ducts.

- Ureter (Option 2) is a tube that carries urine from the kidney to the bladder.

- Cortex (Option 3) is the outer part of the kidney where glomeruli and portions of the nephrons are located.