To solve this analogy, the user needs to understand the relationship between water and glass. Water is typically contained within a glass, so the relationship is one of containment or enclosure. Therefore, the word that fills the blank should have a similar relationship with the word "letter," which is the second term in the analogy.

Now, let's go through each option and explain why it is right or wrong:

A. Envelope: This option is correct. Just as water is contained within a glass, a letter is often contained within an envelope. Therefore, A is the correct answer.

B. Stamp: This option is incorrect because a stamp is not related to a letter in the same way water is related to glass. A stamp is used to send a letter, but it's not a container for a letter.

C. Pen: This option is incorrect because a pen is not related to a letter in the same way water is related to glass. Although a pen is used to write a letter, it does not enclose or contain the letter in any way.

D. Mail: This option is incorrect because mail is not related to a letter in the same way water is related to glass. Mail is a general term that can refer to a variety of things, including letters, packages, and advertisements.

The Answer is: A. Envelope