When cells are in a (n) ___

solution, they are maintaining homeostasis

When cells are in an isotonic solution, they are maintaining homeostasis.

Explanation:

"Homeostasis" is the process to maintain a stable and relatively constant internal environment. In addition to temperature it is maintained for many other factors too. "Isotonic solution" comprised of two solutions having the similar osmotic pressure across a semipermeable membrane.

Such solution maintain homeostasis of a cell via diffusion process. As in such solution the overall concentration on both sides of the membrane remains equal. The cell membrane command the movement regrading entrance and exit of anything in cell, the membrane control water and ion transport because of its phospholipid bilayer structure.