Final answer:
The correct answer to the student's question is homeostasis, which is the state where the human body's systems are in equilibrium, maintaining a steady internal environment.
Step-by-step explanation:
The state in which all body systems are functioning smoothly and in equilibrium is known as homeostasis. Homeostasis is a condition where the human body or any biological system is maintained in a more-or-less steady state. It involves a dynamic equilibrium that is continuously adjusting to the changes the body experiences to ensure that body functions remain stable and within a certain acceptable range. For example, the circulatory system works to carry oxygen throughout the body, remove wastes, deliver nutrients, and regulate body temperature as part of maintaining homeostasis.