What one of the five makes the best comparison?

If you would look at the choices, you may have specific available options. You may think that the stamp is ideal because you need that to send the letter but remember that the comparison states that milk is to glass. Where are you going to place milk? You are going to put it on the glass.

This means that you also have to choose where you are going to place the letter. Book is not a good option too because you will not always put a letter in between the pages of the book.