The correct option is B Informal
The given letter is to be written to a friend, Andy.
Option B: Informal letters are written to friends, relatives and family members, people who are a part of your personal life. Hence B is the correct option.
Option A: Formal letters are written to those whom you don't know, or whom you have to communicate with for work-related matters.
Option C: Semi-formal letters are written to those whom you know and respect, like your senior at work, your teacher, mentor, etc.
Option D: Personal-business letters are written by an individual to a company or an organization.
These types of letters are not appropriate for a friend.
Hence options A, C and D are incorrect.