Final answer:
The appropriate format for a professional business letter is block formatting with left-justified addresses and a typed body. The closing should be brief and can be 'Sincerely' or 'Thank you.'
Step-by-step explanation:
The appropriate format for a professional business letter is to use block formatting. This means that everything should be left-justified, and the letter should be single-spaced with double spacing between each paragraph. The first line should be the sender's address, followed by the date and then the recipient's address, which should be left-justified. The greeting should use the addressee's title and last name only. The body of the letter should be typed using a professional font like Times New Roman or Arial. The closing should be brief and can be either 'Sincerely' or 'Thank you,' followed by two lines left blank for the handwritten signature and typed name.