Economists refer to the activity that takes place in the markets as? a. voluntary exchange b. choice c. opportunity cost d. scarcity.

I believe the answer is: A. Voluntary exchange

The word voluntary exchange come from the efforts that people make to fulfill their needs.
 For example, i am an accountant that couldn't cook fried rice, but i have a lot of money from my salary.
Tina is a cook that could cook fried rice and needed money to pay for her children's school.

So, we decided to voluntarily exchange small amount of my salary with the fried rice so both of our needs are fulfilled.