Answer:
The answer is D. p = 3.99b; $51.87.
Step-by-step explanation:
In this case first we have to represent the equation and then find the total price of the books in order to find the correct answer. This is:
Total price = p
Number of books = b
Cost of each book = $3.99
This is a case of multiplication were we have to multiply the number of books by the cost of each one to find the total price, this is:

Having this we can calculate the total price of buying 13 books, this is:

The total price of buying 13 books at $3.99 is $51.87.
The answer is D. p = 3.99b; $51.87.