What is one of the functions of the kidneys?

The kidneys play a crucial role in regulating body fluid and composition, which includes maintaining the balance of electrolytes, fluid levels, and acid-base homeostasis. They achieve this through processes such as filtration, reabsorption, secretion, and excretion. By filtering blood to create urine, the kidneys remove waste products and excess substances while conserving essential ones like water and electrolytes. This regulation is vital for overall homeostasis, helping to ensure that the body's internal environment remains stable despite changes in external conditions.

The other choices do not accurately reflect the functions of the kidneys. While insulin is a hormone involved in glucose regulation, it is synthesized in the pancreas, not the kidneys. The kidneys do not have a role in storing glucose; that function is primarily carried out by the liver and muscles through glycogen storage. Additionally, bile production, which aids in digestion, is a function of the liver, not the kidneys. This emphasizes the unique and essential role the kidneys play in maintaining bodily functions, particularly related to fluid and electrolyte balance.