The correct options are
A business
B formal
The correct answers are Option A and Option D. Business letters are
generally used to communicate with customers, clients, etc. Here the writer is addressing to someone thanking them for their suggestion in a very polite and professional way. The language
used in a business letter is very professional and formal and thus they
can be categorized under 'Formal letters' as well. Options B and C are
incorrect as the language does not adhere to their formats.