Final answer:
The average healthy adult requires about 2500 mL (2.5 liters) of fluids per day to maintain fluid balance, which matches the daily water output. This includes all forms of water intake, such as beverages, food, and metabolic production.
Step-by-step explanation:
The fluid balance in an average healthy adult is maintained by regulating daily water input to match water output. Typically, an adult will take in about 2500 mL (2.5 liters) of fluids per day, which includes water produced from metabolism (about 230 mL) and water ingested through drinking and eating. This volume matches the daily water output, with the kidneys playing a crucial role in adjusting this balance by conserving or expelling water as required.
According to the Food and Nutrition Board of the Institute of Medicine (IOM), the Adequate Intake (AI) for water for adult males is 3.7 liters and for adult females is 2.7 liters, including water from all sources. Therefore, the correct answer to the student's question about the required amount of fluid per day to maintain fluid balance is that it should match the average water output, which is about 2500 mL or 2.5 liters per day, implying the answer is none of the given options as they are all expressed in mL/kg/day which is not directly comparable to the total mL per day discussed here.