The correct answer is "a steroid hormone that reduces the amount of fluid excreted in the urine".
Aldosterone is a steroid hormone produced in the adrenal gland, whose main function is the homeostatic regulation of the blood pressure, plasma sodium, and plasma potassium levels.
Aldosterone's main mechanism of action is to regulate the reabsorption of sodium and excretion of potassium in the kidney, which in turn influences the retention of water and the volume and pressure of the blood. Based on all this, aldosterone reduces the amount of fluid excreted in the urine.
