The correct option is D Acceptance letter
The correct subject for the given letter is Option B. "Acceptance letter" at a glance sums up the purpose of the letter. Option A is incorrect as it makes a statement which is generally a part of the body of the letter. Option C is incorrect as it does not clearly signify the response, whether the offer is accepted or rejected. Option D is incorrect as 'Thank you for your offer' does not adhere to the format of a formal letter as the purpose is unclear.