Concept:The kidney is made up of many tiny structural units that work together to filter blood and form urine.Explanation:The nephron is the smallest unit that can carry out all the kidney’s functions.Each nephron filters blood, reabsorbs useful substances, and produces urine.Parts like the Bowman’s capsule, glomerulus, and loop of Henle are simply different sections found inside a single nephron.Since the nephron as a whole performs osmoregulation and urine formation, it is the basic functional unit of the kidney.Answer:A. nephron