Final answer:
The correct equation representing the cost function for the bookstore, incorporating both fixed costs and variable costs, is C = 12n + 90.
Step-by-step explanation:
The equation representing the cost function for the bookstore is B. C = 12n + 90. The cost function is made up of the fixed costs and the variable costs. The fixed cost is the cost that does not change regardless of the number of books sold, which in this case is $90 per day. The variable cost is the cost that varies with the number of books sold; the store spends $12 for each book it sells. Therefore, for every book sold, the bookstore incurs a cost of $12. If n represents the number of books sold, then the total variable cost is 12n. Adding the fixed cost to the variable cost gives us the total cost, C = 12n + 90.