Formal letters begin with either a general salutation: Respected Sir/Ma'am (when the name is not known) or by using 'Dear': Dear Mr./Miss X.
Semi-formal letters too begin with 'Dear', since such letters are written to someone whom you know and respect.
Hence, both options A and D are correct.
Option B: Informal letters are written to friends or relatives. While addressing a close one, you don't call them by the title and surname 'Mr. Jones'. Hence, B is incorrect.
Option C: Business letters are generally strictly professional and do not use 'dear'. Hence, C is incorrect.