A formal letter is used for official and professional communication.
It is written using a specific format and tone, and is typically directed to people in authority, institutions, or organizations.
It does not use casual language or personal greetings.
Therefore, it is not appropriate to write a formal letter to a Friend, Uncle, or Neighbour, as these are personal relationships.
On the other hand, a Principal holds a formal position in a school or college.
Students often write formal letters to the principal for reasons such as leave applications, complaints, permissions, or official matters.
This makes the principal the most appropriate recipient for a formal letter.
Thus, the correct answer is (C) The principal.