Introduction:
When writing a letter, it is important to include the address of the recipient to ensure that the letter reaches the intended person. This is a basic rule of letter writing and is essential for proper communication.

Explanation:
The correct answer to the given question is option 'A', which states that we have to write the address to whom the letter is posted. Let's understand why this is the correct answer:

1. Purpose of the address:
The address of the recipient is written on the envelope or at the top of the letter. This helps the postal service in delivering the letter to the correct person. Without the address, the letter may end up getting lost or delivered to the wrong person.

2. Format of the address:
The address should be written in a specific format. It typically includes the recipient's name, their house number and street name, the name of the city or town, the postal code, and the country (if sending internationally). The format may vary slightly depending on the country's postal system.

3. Importance of accurate address:
Writing the correct address is crucial for the successful delivery of the letter. If any part of the address is missing or incorrect, it may lead to delays or non-delivery of the letter. It is essential to double-check the address to ensure that it is accurate.

4. Other components of a letter:
In addition to the address, a letter may include other elements such as the sender's name and address, the date, a salutation, the body of the letter, a closing, and the sender's signature. These components contribute to the overall structure and professionalism of the letter.

Conclusion:
In conclusion, when writing a letter, it is important to follow the basic rules of letter writing. One such rule is to include the address of the recipient to ensure that the letter reaches the intended person. Writing the address accurately and in the correct format is crucial for the successful delivery of the letter. Therefore, option 'A' is the correct answer.