To determine the revenue function for the bookstore based on the information provided, let's break down the details step-by-step:
1. Costs and Expenses:
- The bookstore incurs a fixed daily cost of [tex]$75 to remain open.
- Additionally, the bookstore spends $[/tex]10 for each book it sells.
2. Selling Price:
- The bookstore charges [tex]$15 for each book it sells.
3. Revenue Function:
- The problem defines revenue specifically as the income generated from selling books, not accounting for costs or expenses.
- Therefore, to find the revenue function, we focus on how much money the bookstore makes purely from sales.
Since the bookstore charges $[/tex]15 per book, if [tex]\( n \)[/tex] represents the number of books sold, the revenue [tex]\( r \)[/tex] can be expressed as:
[tex]\[ r = 15n \][/tex]
Thus, the correct equation that represents the revenue function for the bookstore is:
[tex]\[ \boxed{r = 15n} \][/tex]
Hence, the correct choice among the given options is:
[tex]\[ \text{D. } r = 15n \][/tex]
