What characteristic of life best describes the process of homeostasis?

The best example that describes the homeostasis is the control of body temperature!

homeostasis is the property of a system in which a variable (for example, the concentration of a substance in solution, or its temperature) is actively regulated to remain very nearly constant. This regulation occurs inside a defined environment (mostly within a living organism's body).

as relation to the organism body:

the endotherms and ectotherms are two basic group of organism which is related to homeostasis! the organism that can control its body temperature is endotherms and so we can say it can have the ability of homeostasis!