Answer:
No, Luz is incorrect. Marta's quantity demanded has decreased, but her demand has stayed the same.
Explanation:
For $15 per book, the quantity demanded was 4 books per month.
When the price increases to $20 per book, the quantity demanded fell to 3 books per month.
This shows a decrease in the quantity demanded. A change in demand occurs when the price is constant and quantity demanded changes because of change in other factors. But here the other factors are constant and the quantity demanded is changing due to a change in price.
So, Luz's statement here is incorrect.