Explanation:

- Nephron:

- The correct answer is Nephron.

- It is the structural and functional unit of the kidney.

- Each nephron filters blood, removes waste, and forms urine.

- It consists of parts like the glomerulus and tubule.

- Medulla:

- Part of kidney’s internal structure.

- Contains renal pyramids where urine is collected.

- Ureter:

- Tube that carries urine from the kidney to the bladder.

- Not part of the kidney's structural unit.

- Cortex:

- Outer layer of the kidney.

- Holds most nephrons' components but isn’t the functional unit itself.