Final answer:
Money costs are opportunity costs because when you spend money on one thing, you are giving up the opportunity to purchase other goods and services.
Step-by-step explanation:
The correct answer is d. both a and c.
Money costs are opportunity costs because when you spend money on one thing, you are giving up the opportunity to purchase other goods and services. For example, if you spend $100 on a new video game, that $100 could have been used to buy a different video game or other items. Money costs are not always the only relevant opportunity costs, as there may be non-monetary costs involved as well.