The first consideration when writing a letter is the recipient and purpose. The correct option among all the options given in the question is option "A". Actually the first thing to consider is the purpose of the letter and then the recepient. Depending on the purpose the letter would be a formal or an informal letter. The structure of formal and informal letter is totally different. The format of the letter also changes depending on the person to whom the letter is addressed. Grammar wise the letter should not vary much as letters should always be written in correct grammatical format.