Answer:
a. An opportunity cost
Step-by-step explanation:
A trade-off is losing one property or ability of a product while gaining another property or ability.
Production possibility refers to the number of products that can be made with current resources available.
The opportunity cost is the cost of the most valuable alternative from a list of options where one has already been chosen. For example if you are going to watch a movie and if you weren't the next best option for you would be reading a book. So, the opportunity cost of watching the movie is not reading a book.