The human kidney:
A. Concentrates the urine by actively transporting water out of the filtrate.
B. Produces more dilute urine when the collecting ducts become less permeable to water.
C. Responds to the anti-diuretic hormone by increasing urine output.
D. Gets rid of urine from the body by secreting it into the descending arm of the loop of Henle.

Hint: Human kidneys are organs that lead to the release of excretory waste about the body, they have functional units known as the nephrons which carry out this process of excretion employing filtration, reabsorption, and secretion.

Complete answer: Human kidneys are responsible for the release of excretory wastes from the body in the form of urine.
1. The filtrate undergoes various steps of secretion and reabsorption before it is released as urine from the kidneys.
2. The concentration of this filtrate from the glomerulus is similar to that of the blood plasma.
3. This then in the convoluted tubules of the nephron undergoes a process of tubular reabsorption to form concentrated urine.
4. This process of urine concentration is achieved by actively absorbing the water from the filtrate formed by the glomeruli.

Additional information: The nephron is the structural and functional complex unit of the kidney. Each human adult kidney consists of around 1 million nephrons, while a mouse kidney contains about 12,500 nephrons.
The kidney plays role in the control of the volume of various body fluids, fluid osmolality, acid-base balance, various electrolyte concentrations, and removal of toxins.

Hence, the correct option is (A), Concentrates the urine by actively transporting water out of the filtrate.

Note: For excretion and osmoregulation by the kidneys the process of filtration, reabsorption, and secretion is very important.
Also, the countercurrent mechanism plays an important role in the concentration of urine.