Final answer:
In this phrase, the correct choice is 'por' to express the percentage.
Step-by-step explanation:
The correct choice in this phrase is por.
The word por is used to indicate the means or method by which something is done. In this case, it is used to express the percentage or rate of 40%.
Example: Pagué $50 por la camiseta (I paid $50 for the shirt).
Learn more about How to choose between 'por' and 'para' in Spanish