Which of these actions isn’t part of your body’s homeostasis?
a. going to the bathroom
b. smiling when you are happy
c. breathing harder when you exercise
d. sweating when you are hot

B. smiling when you are happy

This is not an action that is controlled by the process of homeostasis in the body. Homeostasis is the process which is responsible for regulating the the internal environment of the body.
for instance, you go to the bathroom when your kidneys want to eliminate excess water and other substances.
Your lungs need more air when you are exercising so homeostasis is responsible of seeing that you get enough of it.
and sweating when you are hot to remove excess heat from your body via sweat.
These are all involuntary actions controlled by the process of homeostasis.
Smiling is a voluntary action. you do not need to smile. and you can whenever you like and not when you don't.