Topic: Composition,

Best Answer Explained in Below-:

In understanding the structure of a letter, particularly a formal one, it is important to recognize that it typically consists of six primary parts:

1. **Heading**: This includes the sender's address and the date, providing context for the recipient.

2. **Inside Address**: This section contains the recipient's name and address, establishing who the letter is intended for.

3. **Salutation**: A polite greeting to the recipient, such as "Dear [Name]," or "To Whom It May Concern."

4. **Body**: This is the main content of the letter where the purpose of writing is detailed, including any relevant information or requests.

5. **Closing**: A courteous way to end the letter, such as "Sincerely," or "Best regards," followed by a comma.

6. **Signature**: The sender's handwritten signature followed by their typed name, which personalizes the communication and provides authenticity.

These six parts together create a coherent structure essential for effective communication in a letter, which is why the correct answer is D: six.