The correct answer is A
The books sold are 30,000, so that is our total sale for January. His sales are decreasing by an average of 500 books per month, which means he is losing sales so it is a negative value, and we multiply that by x (number of months) since the question has not stated the number of months. Therefore, the equation would be f(x) = 30,000 - 500x
Hope this is of help!