The correct options are
A Dear Sir/Madam,
B Dear Mr./Ms./Mrs. ...
C Dear Customer,
The correct salutations are Options A, B and C. In formal letters the salutations generally include 'Sir/Madam' or they may include the full name of the receiver starting with a 'Mr./Ms./Mrs.', as suitable. Since this is a business letter the salutation may also include 'Customer' instead of any of the above. So, all the three options are correct and suitable according to the format. Option D is incorrect as the first name is generally used to address someone who is well known to the writer and so is used in an informal or a semi-formal letter.