What is the economists interpretation of costs? What does the following statement mean: the concept of cost

Question:

What is the economist’s interpretation of costs? What does the following statement mean: “the concept of cost is far richer (pardon the pun) than the dollars and cents you hand over at the cash register.”?
a. Costs are plus sunk costs.
b. Costs are what must be given up to gain something.
c. Costs are the value of the opportunity costs of employees.
d. Costs are direct and overhead.
e. Costs are direct, overhead, and sunk.

Opportunity Cost
Opportunity cost is the profit lost when one alternative is selected over another. The Opportunity Cost refers to the expected returns from the second best alternative use of resources that are foregone due to the scarcity of resources such as land,...