Answer:
d. people face trade-offs
Step-by-step explanation:
Monica has to choose between two options. If she chooses one, she forgoes the other option. This is known as trade off.
Economic agents face trade offs because resocurces are limited and wants are unlimited.
In the question, Monica has limited resocurces which is money. She has just $500.
But has wants that exceeds her available resources.
I hope my answer helps you