Answer:
The correct answer is (A) the output effect is stronger than the price effect
Explanation
The price elasticity determines which effect is stronger.
In this case,
Marvin reduces his price from $25 to $20, the quantity sold increase and total revenue increase by $40.
As the revenue increase when price goes down, we can say we have a elastic demand. Also, if we have an increase of revenue, that means that the output effect is stronger than the price effect