Answer:
C. She wants to show by what percent the book was discount.
Explanation:
We are given that,
The expression showing the percent of the original cost =

Re-writing the expression, we have,
The expression showing the percent of the original cost =
=

So, we get the total price 'b' is reduced by 30%.
Thus, the expression shows the discount she received on the book is 30%.
Hence, Jessica re-writes the expression to show the percent by which the book was discounted.