The correct answer is option 3, the kidneys. The kidneys are responsible for regulating water balance in the human body. They play a vital role in maintaining the proper levels of water and electrolytes in the blood. The kidneys filter waste products and excess water from the blood, which are then excreted as urine. They also reabsorb water and certain nutrients from the urine back into the bloodstream, helping to maintain a balance of fluids in the body. The kidneys are highly efficient in their ability to adjust the amount of water excreted or retained based on the body`s needs, such as during periods of dehydration or excessive fluid intake. While the heart, liver, and lungs all have important functions in the body, they are not primarily responsible for water balance.