Politely expresses sadness, regret, or appreciation are not services provided by a properly written business letter. Option A is correct.
Business letters are formal paper developed to communicate between, to or from businesses and generally sent through the Post Office or by courier. Business letters are mainly intended to inform, instruct, request, inquire, remit, order, advice, correct and to question.
A business letter constitutes usually a letter from one company to another, or between such organizations and their customers, clients and other external parties.