Answer:
b. $10
Explanation:
She was willing to pay $50, which would've then been gone, but she'd have had a new soccer ball.
But when she got to the store maybe they were on sale for $40. Now $40 is gone, she has a new soccer ball, AND she still has $10 in her pocket that she brought with her to buy it.
Sounds like a "consumer surplus" to me.