Hello!
3 and 20 are the number of stamps she has, but she isn't obligated to use them. She also does not have $20 stamps either as the top two suggest.
We can add .75x and .45x to get at least 5.7. We also know that at most she can use three of her .75 stamps, and at most 20 of her .45 stamps. Therefore, this matches answer D the best.
Therefore, our answer is D.
I hope this helps!