We assume the best option is the cheapest option, so we need to calculate the total price for each option:
A) $0 + $15×9 = $135
B) $20 + $10×11 = $130
C) $10 + $5×24 = $130
D) $5 + $10×15 = $155
So unless there was a typo in your problem, either of option B or C will be the best option since they are the cheapest at $130.
If you can only select one answer, it becomes a matter of preference. Would you like to pay off the microwave quicker, then choose option B.
If you would like to have more time to pay off the microwave, then choose option C.
Hope this helps!