The correct answer is B - the writer's identity.
Imagine if you received a letter, and you start reading it, but you have no idea who is it from! It would be confusing. The day, who you are, and your information will be located at the top of the letter, then include the name and information of the person receiving, in the same format.
Example:
(Insert your name)
123 ABC St.
New York City, NY 99999
April 1st, 2017
Dr. Patricia Brown
777 West Ave.
Los Angeles, CA 111111
Dear, Dr. Patricia Brown,
(Purpose for writing goes here)
(An expression of thanks)
Sincerely,
(Insert your name)