* Homeostasis is the ability of an organism to maintain a stable internal environment despite changes in the external environment.

Let's break down the options:

* A. a chicken laying eggs: This is a reproductive process, not a direct response to maintaining a stable internal environment.

* B. a person sweating on a hot day: Sweating is a mechanism to cool the body down when it's hot. This helps maintain a stable internal temperature, which is essential for homeostasis.

* C. a lizard changing color to avoid predators: This is a camouflage strategy for protection, not directly related to maintaining a stable internal environment.

* D. bacterium dividing into two: This is cell division, a process of growth and reproduction, not a direct response to maintaining a stable internal environment.

Therefore, sweating is a clear example of an organism (the person) using a mechanism to maintain a stable internal temperature (homeostasis).