Opportunity costs refer to:

a. current spending habits.
b. changing economic conditions that affect a person's cost of living.
c. storage facilities to make financial documents easily available.
d. trade-offs associated with financial decisions.

i think is c. or d.

The correct answer is D. Trade-offs associated with financial decisions because the opportunity cost is what you give up in exchange for something else. For example: If you must choose between a hamburger or a hot dog, the one you do not choose is the opportunity cost.