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.

C) A group formed to influence government policy

Explanation:

Interest group is also called special interest group or pressure group is the group of individuals or organizations which is formally organized and it is based on the basis of one or more shared concerns. This group attempts to influence public policy in its favour.  In US the examples of Interest group are the U.S. Chamber of Commerce and the National Association of Manufacturers (NAM.