The correct option is A formal
The given sentence is most appropriate for a formal letter.
Option A: Formal letters are written in a formal language, the English used must be accurate and the formation of sentences are longer and complex. The given sentence praises an tutoring class, possibly by a student. This situation is closest to a formal communication.
Hence option A is correct.
Option B: Informal letters are written to friends and relatives. Their tone and style is casual, and the English structure is not of utmost importance. The given line doesn't seem to have been written to a friend. (The tone would be: A lot of people have told me that they are very happy with your classes and your articles. I am so happy to hear that!)
Hence option B is incorrect.
Option C: Semi-formal letters are written to people whom you know and respect. For e.g., teachers, mentors, seniors. This line does not sound suitable for any of these people.
Option D: Business letters are written by an individual to a company or organization, for business and other work-related purposes.
This line does not relate to business. Hence D is also incorrect.