What term describes the maintenance of proper water balance and dissolved substances in an organism?

The term that most accurately describes the maintenance of proper water balance and dissolved substances in an organism is osmoregulation. This is a specific physiological process by which an organism regulates the concentration of solutes in its body fluids, thereby controlling water balance. Osmoregulation involves mechanisms that help organisms maintain their internal environment despite changes in external conditions. This is crucial for cellular functions and overall homeostasis.

Homeostasis is a broader term that encompasses the regulatory systems that maintain stable internal conditions, including temperature, pH, and electrolyte levels, in addition to water balance. While hydration refers specifically to the levels of water within an organism, and fluid balance pertains to the equilibrium of fluids within the body, osmoregulation focuses specifically on the control of solute concentrations and water movement. Thus, osmoregulation is the most precise term for the process described in the question.