Answer:
Trade offs
Step-by-step explanation:
Trade-off is a sacrifice made to have a particular product.
Economic agents face trade offs because wants and needs are unlimited while resources needed to satisfy them are limited.
In this question, the resources are $120 while the wants are guitar and amplifier.
$120 isn't enough to buy both items. Billie Jean has to choose one of the options. If he chooses the amplifier, he forgoes the opportunity of buying the guitar and if he buys the guitar, he forgoes the opportunity of buying an amplifier. This illustrates a trade off.
I hope my answer helps you