As an economics instructor, let me help guide you so you can figure this one out. The KEY here is understanding what opportunity cost means.
Remember, it's what you sacrifice! So here, you have a choice of spending money right now or investing it and making money later on.
Let's assume you spent the money now. That would cost you $100 but give you immediate satisfaction.
Now let's assume you put that money into the bank? Would you get immediate satisfaction? No! And that's because you have to wait until you get your interest.
But will you make money by putting it into the bank? Yes! So your opportuntity cost of spending that $100 would be what you'd lose had you put it in the bank and gotten that interest.
Now all you have to do is figure out how much interest you would have earned!