Buehlermaci,
The answer is option A "scarcity." It's not option B because economics is the distribution or a production of a item. It's not option C because opportunity cost is when you are looking at the value of two things before you make a decision on which you want kinda like pros and cons. Scarcity is the shortage on a resource but the person decides to spend it on wants and needs so basically the difference between limited resources and wants and needs.
Hope this helps!