Your health club is offering a free one-year membership for the person who attends the most yoga classes in March. This results in more people attending yoga. Which economic principle does this statement best represent?

The economic principle is "people usually exploit opportunities to make themselves better off".

Explanation:

Here, the health club is offering a free one-year membership for the person who attends the most yoga classes in March.

So, there is an opportunity for every individual to get a free one year membership. So, more and more people will try to attend more yoga classes in order to obtain free membership for one complete year. This choice will be made by most of the individuals because they want to get rid of paying fee every month. Hence, this will tend every individual to grab and exploit that opportunity to make themselves better off.

Thus, the increase in people attending yoga classes is based on the economic principle "people usually exploit opportunities to make themselves better off".